Decontamination Chamber: Fortunately, these days, it just involves a light mist of nanites checking you out (while clothed, so no fanservice, alas), but it’s routine protocol for (physically) entering a new habitat or, indeed, planet, given the risks of disease and, worse, ecological contamination.
It probably tells you everything you need to know about the Empire that you can bring all the weapons you like along on interstellar trips, but gods help you should you smuggle a single unauthorized microbe…
Alien Hair: Sometimes. Also, sometimes, fur, tentacles, symbiotic fungi, cilia, etc.
Alien Geometries: Something seen in automation-dominated infrastructure areas, where buildings and systems which are – for the most part – only ever used by AI tend to be made in complexes of simple geometric shapes, lines, and curves, without any of the softening or decoration that most sophont eyes find comfortable.
Alien Blood: Played straight, in many varieties. For example, eldrae blood is close to indigo in color, due to the not-hemoglobin it uses (borrowed from Elieran bluelife, which is indeed mostly blue; and it would lead to noticeably different skin tones if they weren’t all so damn pale anyway). Kaeth blood is silvery-white, and notably conductive. Myneni “blood”, really crystalplasm, is whitish and feels sandy. Esseli blood is red-orange, and mirilasté blood is a fluorescent magenta.
The Aesthetics of Technology: Oh, the Imperials play this one completely straight. After all, the Imperial motto is Order, Progress, Liberty, and it would never do to make all those neophilic people think that they weren’t getting their Progress, capital-p included. So not only should it be all future-y, but it should look that way, too.
Which is why the stereotypical piece of technology looks like an Art Deco iPad rendered in organic shapes, out of burnished metal and crystal and gleaming stone and oiled wood, with some probably unnecessary glowy bits added around the side, and generous use of trigraphic (“hologram”) interfaces displaying all the information you need and quite a bit of information you don’t need but might quite like to have anyway. Larger industrial machinery takes this and adds a few tons of brass piping on top. Just to really take the futurity up to eleven, most of it is happy to respond to thought commands without you actually having to use a physical interface at all, although – especially on the large industrial machinery – it’s present anyway just in case you need it, and because it wouldn’t look machine-y enough without it.
And it’s all polished until it gleams, because it’s supposed to look beautiful as well as impossibly high-tech. These chaps know their market.
Action Bomb: Spider mines. Also AKVs (Autonomous Kill Vehicles) who can use themselves as a kinetic-kill weapon when they run out of ammunition. Of course, they back themselves up first, so it’s not like they’re blowing themselves up permanently.
Abusing the Kardashev Scale For Fun and Profit: Working their way up to this right now, the Empire has a couple of Dyson spheres, one under construction, the other completely dedicated to energy generation, plus other energy-generation facilities scattered here and there and about the place. This makes them, I believe, a solid Type II, although given the constraints of light speed, it’s going to take them a while to work up to III.
Absurdly Spacious Sewer: Not sewers, technically, but Imperial cities are known for their lengthy networks of service tunnels running under the streets, through which the actual pipes and cables that supply water, sewage & garbage removal, power, nanoslurry, bandwidth, and other utilities are routed, along with the pneumatic mail system and the tracks for delivery robots. (They would much rather dig the street up once – if a lot more – when they’re building it than have to dig it up again every time some individual system needs poking.) And you can, if you have the right master keys and don’t mind dodging the security systems, get just about anywhere you please via the service tunnels.
Well, except into people’s homes and other private buildings. It’s not like they don’t have basement doors down there just as secure as their outside doors are upstairs.
Also unlike the sewer passages, the service tunnels – after all, the sewer is a sealed pipe inside the service tunnel – are well-ventilated, generally quite clean, and kept reasonably free of vermin. Utility people have to work down there a fair bit, after all – chambers along the way hold various distributed bits of equipment.
(The storm drains and their sumps are further down, below all of these, the Underways, and the subway tunnels. Everything’s got to drain somewhere, right?)
Abandon Ship: As mentioned under Escape Pod, only occasionally useful in the physical sense due to the lack of places the escape pods could take you, given their delta-v constraints – nor are they likely to have enough delta-V to correct a sunward plummet, or other highly inadvisable course – and the likelihood that if something is going to explode badly enough to destroy the hulk, it’ll catch any conceivable pod-launch in the blast, too.
No, abandoning ship is something done by mindcasting – i.e., transmitting the minds of everyone on the ship somewhere else (some vessels carry a passel of emergency tangle for exactly this purpose). The copy of you left on the ship gets to go down with it – and, actually, in more than a few cases, that the copying nature of mindcasting lets you both abandon your ship and go down with it at the same time is seen as a positive advantage, even if dying liners exercising this option prefer to break out a round of euthanasia pills for the passengers, at least.
Escape Pod: Averted. Fusion reactors don’t explode, they fizzle. Antimatter reactors (well, really, the storage cryocels) do explode, but in general, there’s no way you’re going to get away from them fast enough in order to matter. And unless you’re in orbit around a planet or co-orbiting with a habitat cluster (in which case there are almost certainly plenty of people around with small craft anyway), you’re not going to be able to get anywhere with the delta-v that can be packed into an escape pod, so you might as well stay with the hulk of your ship, however shot up it is and whatever resources it represents. Escape pods are (mostly) pointless.
And even in the military scenario when you want to get to a different ship, Dodge for the getting out of, that’s what mindcasting is for.
Animate Inanimate Object: Not to the full extent of this trope, but due to the impact of ubiquitous computing and the generally spime-ish and MEMS-ized nature of Stuff, it’s true enough to make the animists happy.
Location Theme Naming: Sometimes. There’s a class of battleships named after Imperial habitable planets, a class of oilers named after gas giants, a class of cruisers named after Elieran mountains, a class of frigates named after well-known drift-habitats…
Ancient Conspiracy: Averted. Because, well, yes, ancient cabals, except there are thousands, maybe millions of them. And they’re not terribly shadowy, at least not for any value of shadowy that precludes web sites, press releases, and such. And the course of history seems to muddle through just fine doing its own thing. But apart from that…
Loads and Loads of Races: Played very, very straight. While I have by no means detailed all of them, there are canonically hundreds of different species in the Associated Worlds, each with its own set of races (a few) and clades (if any, many). It’s a life-rich galaxy out there.
Ancient Astronauts: Almost certainly happened in at least a few times and places, for the same the-Galaxy-is-old-and-relatively-life-rich reasons that are why half the systems in it are practically knee-deep in elder-race trash.
Also, technically, connected with the origins of the eldrae and greenlife in general from prehistoric Earth, although given that Earth is not found and not likely to be in canon, no-one has a clue where those fossils of so-called Pseudoeldrae archaea came from, or exactly what they have to do with anything anyway, even if we’d clearly recognize an offshoot of genus Homo if we saw one.
Averted in the “alien abduction” meme sense, just because, as mentioned, lighthuggers ain’t that subtle.
Lizard Folk/The Reptilians: About the closest you’ll find are either the kaeth (bipedal, vaguely draconic/saurian, and also from such a metal-rich world that their scale-analogs are actual metal) or the mirilasté (legged serpents). Both defy the traditional reptilian-appearing race stereotype by being civilized and urbane, although the kaeth do add violent to that. Also, both are younger races.
Amazon Brigade: For historical reasons having to do with the local units they used to be, the Empire has several Legions that meet this trope (and, for that matter, its diametric opposites (yes, opposites; not every species has two sexes)), although most legions are mixed. Of course, in-world, no-one finds this the least bit remarkable.
Living Starship: The genesplicing esseli and the residents of Kythera – the Empire’s masters of biotech – are the chief users of this sort of technology in the Imperial context, although they’re by no means the only ones in the Associated Worlds to go for this sort of thing. Although, in deference to certain realities about the capabilities of carbon-based organics vis-à-vis metal and ceramic, the hulls (mostly, although some specially-engineered and composite-laced wood-analogues are an exception), engines, and other high-energy components tend to be not living in anything resembling the conventional sense; i.e., the ships are only semi-organic. (See also, for example, the note under Flesh Versus Steel about the Gardeners of Rechesh; while they do manage to construct entirely biotech spacecraft, their hulls are vulnerable and engines slow compared to virtually anything else anyone’s ever put into space.)
Flesh Versus Steel: Both have their partisans within the Eldraeverse – many branches of the Silicate Tree are steel fundamentalists, although how much of that is rationalized hatred of their organic creator-enslavers is open to question, and they’re not the only ones. Likewise, there are partisans of the purity of the flesh, like the purist Gardeners of Rechesh (who have endless trouble trying to build organic-tech spacecraft, etc.) and more moderate, tool-accepting groups.
The Imperials, contrariwise, would point out that choosing one above the other is to pointlessly limit yourself. Of course, since the average Imperial is wandering around with plenty of “steel” (in the form of billions of technocytes and technosomes, etc.) scattered all through their “flesh”, they would say that, now wouldn’t they?
Living Relic: Quite a few, including some cryonauts and other last-survivor types, but the most common form of these is still-readable archives left behind by dead civilizations. Many of which archives include suspended mind-states – sometimes biological but more often AI – or recipes to create AIs, as part of their contents.
While some of these are, indeed, lights flung into the future or bearers of the ancient wisdom of forgotten civilizations, enough of a minority of them are examples of the kinds of dangers I’ve mentioned under Gone Horribly Right or will later mention under Sealed Evil in a Can that blithely cooking up recipes you dug out of ancient archives is something you should approach with approximately the same caution as poking Cthulhu with a stick.
(Nonetheless, there are major commercial entities – say, Probable Technologies, ICC – who make a living doing it. So it goes. They also make a living selling spin-off technologies with names like “meme-gap archive recovery systems” and “broad-spectrum gigaton-range autodestruct protocols”. So that goes, too.)