EDIT: Seems dynamic music is back in style in some very recent games, many of which I haven't really played yet. Good. 
For me, it's dynamic music, the kind that some games had that adjusted moment by moment to what was happening in the game.
The best-known example of this in the 90s game TIE Fighter, where the moment more enemy (or allied) ships showed up the music would have a little additional flourish to acknowledge the shift in battle. There were pre-battle tension tracks, battle music, complications of battle, grandiose flourishes for the arrival of enemy or even allied capital ships, and victory and failure music all ready to flow into the next seconds of the game.
A lesser-known but still excellent example of this was in Ultima Underworld and its sequel, where drawing a weapon had its own special "preparing for battle" tension music, getting attacked had a jump-out-of-your-skin joltingly sudden musical start that actually scared me as a kid when I got ambushed, music for battles going well, going poorly, victory and defeat.
I wish more games did those sort of second by second musical changes, but they've sort of fallen out of fashion for the most part. 
One of my least favourite things in the entertainment industry is when a new innovative or interesting thing is in an otherwise mediocre/terrible product, its used as a justification to never do it again. Of course, you can go far in the other direction and assume that because something is different means it is "good", whatever that means. I appreciate artistic risk taking tho
There's probably a decent number of random mechanics in RTSes that got used once and then never again. I remember Act of War had the prisoner mechanic, which encouraged aggressive risk taking play. It was present in the sequel but I think they could have leaned into it further. Not that any random mechanic is going to be generally applicable. I always wanted RTSes to be more asymmetric but that's harder for balance and the marketing of "competitive" ranked play.
A mechanic I really enjoyed in a recent game was from Enlisted. It's a competitive (as in, you're playing against other humans) first person shooter, but your respawns are your bot squad. Most of the time when you're shooting at a rando crossing a field, it's a bot. This experience meant that even the worst players were getting kills and having an effect (by killing the better players respawns). I'm not terrible but not that good at FPSes either, but I found the experience way less frustrating overall. The rest of the game was mid. Freemium. But I'd be interested to see that in other games.
I remember an old game called Metal Fatigue, which had a battle layers mechanic. Obviously many city/base builders have had this for ages, but I could totally see it in a modern imagining of the current Palestinian thing. Having a multilayered tunnelling mechanic to fight over.
I think overall when I'm playing non-competitive indie games, my main gripe is usually "I wish this game was the same except for this thing from another game". Modding is generally less available nowadays, and I'm not the most amazing coder so remaking the whole game with the extra mechanic from scratch isn't really an option (probably).
Metal Fatigue also had a nice system of putting mechas together out of individual parts, which you could steal and reverse-engineer from the enemy factions. That game maybe had a few too many mechanics going on, but I had a lot of fun with it.
Yeah, good stuff.
The Goo faction in Gray Goo is cool as fuck, your "base" is just this big mobile amorphous blob that engulfs and digests resource nodes and even enemies that touch it. It can split into more copies of itself or smaller, faster blobs that further divide into your actual specialized units. Everything is a slick-looking liquid metal with glowing cyber hexagons running across the surface and eerie creaking/bending metal sound effects.
Yeah, petroglyph leaned into it for both Universe at War and Grey Goo. I always felt like the games felt kinda junky, but I appreciated the effort in making the factions feel substantially different.
Fun fact: the bloom post-processing effect was popularized by a Gamasutra article by the programmers of Tron 2.0, a game which received mostly middling reviews and has since largely been forgotten.
I'll say that one of the most interesting and enjoyable shooter experiences I've ever had was the beta version of a game called Ace of Spades that was developed by one guy who built the engine from scratch. It was literally just minecraft with guns, but the very limited set of tools (the best gun was a rifle that fired perfectly straight and had no scope) and enormous open spaces resulted in an experience that was both slow and tense.
(Um, I don't know why your post triggered me into writing this pitch for a wishlist game. Maybe the minecraft with guns bit? idk, I got excited)
I have this pitch for a builder game where you're a military procurement/engineering firm. The LoD would be about what Stormworks has (25cm blocks, or maybe 20 or 10 cm), you spend time fiddling around with air fuel ratios and stuff. You'd be able to fiddle with various war nerd numbers on vehicles you create, but there wouldn't be much for you to do with the vehicles directly. Instead, you teach bots how to use the vehicle (some sort of waypointing system, some vehicle tests like turning, acceleration etc etc). After that, your vehicle and usage data is compiled and a little war goes on in the background. Hypothetically, this war would be happening on another screen or you could refer to it. Because the vehicle is compiled into this RTS mode and not run as a physics simulation (or at least, would be run as a very cut down simulation), that section would be quite light. Possibly multiple layers to examine (strategic, operational, tactical). Your vehicles would have logistical strain (e.g. fuel, maintenance/wear, damage from fire etc). You'd probably want to define a few other variables on how its used (e.g. This is a TANK, GENERAL PURPOSE, SWARM or something). I don't think it would be possible for an AI to account for all ways people would design vehicles and use-cases, but the basic classes are pretty standard nowadays, and people could request things that feel plausible to the dev.
A few reasons for doing it this way: Having it so that the vehicle is tested by itself on multiple predictable scenarios means the physics simulation (e.g. denting, beams bending etc) can be more detailed, and allows for more complicated vehicles. Once its "compiled" so that the bots can use it, it will run quite light (this is sort of explored in From The Depths, but not to its fullest extent)
You'd watch combat and take notes on what works well and what does, and work on new designs as the war gets under way. Your new designs that you produce and test would percolate through the logistics system and slowly start appearing on the front.
There'd also be a little thing where you could define your squads that the AI uses in the war (e.g. 12 dudes, 1 command, 2 fireteams, each fireteam has a LAW and 5 assault rifles, command has 1 commander and 2 machine guns etc), with some reference to real world stuff. This would obviously be important for transport vehicles and logistics.
There'd be a mode where you'd have to do it "in real time" (i.e. no pausing for designing), a more freeform creative mode where you can design and save freely without worrying about wars and launch battles with your vehicle instantly, and a thing where you could compile all of your designs into a faction. Presumably, the game would ship with a few real world referenced factions, people could mod in their own ones. And people could also mod in maps that the AI will fight wars on, and opponent factions (of varying degrees of fairness). Tutorial mode, build a truck that carries a squad. It's an electric truck so you don't have to program a gearbox.
It's probably a bit beyond me as a coder (maybe, idk, the primary time I was trying to learn coding was when I had pretty severe depression), but maybe as a fresh godot project if applicable? I think it would absolutely kill amongst a certain sort of war nerd.
General Dynamics: The Game.
Arma has a similar respawn system, if you want it to. Controlling the AI is way, way less streamlined than Enlisted though