See last question. Although that's not surprising, given that they're inspired by Dungeon Keeper.
I actually liked that mechanic though in most games it was fairly useless. Not really sure if it was all that effective in DK though, I hardly ever used it except to go "I'M A FLY IN A DUNGEON YEEEEEEEAH!"
Basically all the mechanics of The Specialists mod.
Another game like Commandos.
Why I mentioned this mechanic was because I think the change in perspective from RTS to FPS helped a lot in terms of immersion. It gave me a real connection to my minions and dungeon. It had some practical applications as well. The great thing I think is that those applications weren't very obvious. Like your example with the Fly.
Possession in Dungeon Keeper was also vital to persuade your stupid Dark Mistress to attack the enemy dungeon heart instead of firing pointless lifedrain bolts that did no damage.
Irrelevant on further examination of the rest of the thread.
The creation/customisation tools of Spore.
The movement of Tribes: Ascend.
Combat from Batman AA/AC (but more challenging [kind of just happened with sleeping dogs])
Eve's player driven economy/world
The world generation of Minecraft/Terraria/Randomisation aspects of roguelikes.
You probably can't put all of these things into one game, but they're the kind of standout mechanics that can get me interested in a game instantly.
Grappling Hooks & Jetpacks have around a 93.47% chance of instantly improving any game.
Brink I thought was going in the right direction with the whole idea of sliding, jumping, and using the enviroment to your advantage. WRONG. That game was a big let down. I tried so hard to like it but lost intrest in a week maybe.
But thats what I would like to see done right.
1. Fun weapons in FPS games. More fluff than mechanic, I suppose, but limiting the selection to bullet shooting gun, rocket shooting gun and grenade lobbing gun is boring. FEAR had the skeletiser, flamethrowers are always nice.
2. The deterioration of the game world as the game progresses. Procedural aging of structures, moss growing up stone walls, weathering of wooden tools; stuff that gives a sense of difference between newly crafted stuff and stuff from 200 years ago besides "you used this axe 300 times and it snapped". A game would need to take place over a pretty long time to take advantage of this though. Maybe a game about gandalf, since he's 8000 years old or something.
The end purpose I'm thinking of is just making backtracking nicer. Retracing your route, but the route has changed.
3. Levels set on multiple complex moving vehicles; ships with different decks, rooms etc. I'm thinking of that level in Far Cry 1 when you're on a sinking fishing boat, or that airship duelling game out soon, or the naval battles in BF1942.
Homeworld's fleet management. Well, there wasnt much to it, just taking care of the sub-capitals by docking them in your mothership or carrier. In the first Homeworld, your sub-caps had a limited amount of fuel, something which was removed in the later games (In-game explaination was that the technology had advanced so that smaller scale fusion reactors could be used by fightercraft), so they regularly had to dock with a carrier or the mothership. Shame, it added some depth to the game, especially during the massive fights with the Kadeshi.
I especially loved watching them all dock up when it's time to leave the area - a huge stately procession of ships, especially in the later levels. Sub-caps moving into a docking queue, capitals moving into formation with the mothership.
Actually, I think it might have been a seamless transition, which was very impressive on the ps2. To the point where you could step out of the chair during a space battle and go cringe in the cargo hold. You'd be a sitting duck and the ship would be destroyed quickly but it was neat that you could do it.
I'm going to have to dig it out just to check now.
Full body awareness
A combination of ARMA's camera where you can see your own shoulders instead of just your legs and the healing of Far Cry 2 where you got a first-person view of your character pulling bullets out of himself and relocating bones would do wonders for player immersion.
More human animations
It broke up the boredom of just walking to a checkpoint seeing Max stretch and move his head around while walking to the strip club in Max Payne 3. He was moving like a real person instead of just a series of animations in a loop and that gave him more personality than his monologues.
When a bullet hit an object it flattens and looses speed, which causes it to start turning vertically. That causes a much larger hole when it exits a body and is part of the reasons guns are so lethal. Sniper Elite V2 nailed this. It's almost mesmerizing to see the bullet tumble out of a person's skull like a real one would. And there's a little round hole where it went in, and big nasty one where it left. Do this more games.
Press X to skip this scene
Sometimes people want to play a game more than once and they don't feel like watching the same cutscenes over again.
If you can see it, you can go there
Seen it in a few shooters before, but never in multiplayer. Why is that? Would it not be incredibly fun to hit a guy's leg and deny him the ability to run because of it instead of it just taking away some of his HP?
I'd like to see Wurm online, but done good (and not clunky & buggy as it currently is). So a game with Terraforming, complete freedom in crafting, cooking, fishing, woodcutting, mining etc and a good survival mechanic (with hunger & thirst). Though I'd like to discard the low fantasy trope and go for a path less travelled so to speak (maybe prehistoric, bronze age, steam age, steam punk, pirates, vikings, explorers or even modern times, though that might be too "Second Life" ;)