My list.
Things I’d like to have been in FO3 from Oblivion…
Detect life.
Using skills to increase them.
Things I’d like to have been in Oblivion from FO3…
Guns!
V.A.T.S.!
My FO3 character.
If Baldur’s Gate 2 had used the turn-based combat system from Fallout 1/2, I think I would have loved it. Instead I found it to be a pretty good game marred by one of the worst battle systems I’ve ever seen.
Actually more game should use F2’s battle system, that was perfection.
I’d like to see the **gem from Thief **that indicates how well you are hidden in EVERY game. Why can no other game but Thief do stealth well? I’ve tried all critically acclaimed Solid Snake games, but it’s a total guess as to whether a baddie is going to see you or not.
You didn’t ask, but I’d also like to NOT SEE scaling-level bad guys ever again. What’s the point of leveling up if the baddies go up with you?!
I always had this dream of driving the Carmageddon car through a round of Counterstrike. 
Castlevania’s Symphony of the Night done using Trine’s graphics and physics.
Medieval Total War in Romance of the Three Kingdoms Style
Okami’s ‘return to you nearest checkpoint’ in any platformer
Okami’s brush stroke system in action games.
Devil May Cry combos in Dynasty Warriors
Knights of the Old Republic’s combat resolution in Fallout 3 (I’m not a fan of FPS)
Final Fantasy 10’s ‘switch to another party member’ combat system. Honestly, what’s the point of giving me a roster of seven characters when I can only field three at a time?
I’ve never played Theif. But the Splinter Cell series of games have done Stealth pretty well in my opinion.
While we’re on the subject of SC. I’d like the sneaking up behind people and breaking their necks in every game (or just holding a knife to their neck if you’re ‘good’)
I’d like to have hockey penalty boxes in basketball. Instead of no-harm-no-foul the rule would be hit-someone-go-sit-a-while.
I’d like the stop clock of timed chess matches to work for golf. Stop lining up the putt already or lose the hole.
I’d like soccer to have helmets.
Now, see, to me this makes no sense. If you can switch out anyone at anytime in battle…why doesn’t everyone join in at once?! From a logical “real world” standpoint, why would the other four team members just stand around while you fight a a large creature things, just waiting and hoping you ask for their help, only to replace someone else?
I think Chrono Trigger had the best explanation for why you could only have three people at a time. Any more than that will destabilize the time travel wormholes.
You should try “Temple of Elemental Evil”, then. Pure turn based D&D. Of course, story wise it’s got nothing on BG2, it’s pretty much one big dungeon crawl. Still, cleaning a rooms with a perfectly placed fireball while your tank holds up the door is pretty sweet 
Or, of course, Arcanum - Fallout-like combat system, Fallout looks, and steampunk ! I wish there was more steampunk out there.
I’d like an RPG style inventory management system, such as was used in STALKER or Planetside, in more FPSs, especially the slower paced tactical ones such as Crysis or the Far Cry series. Obviously, not all… it wouldn’t be a good fit for a game such as Team Fortress 2 with its incredibly fast pace.
I would like more games to have Diablo 2 style randomized levels, enemies, and gear, so that each playthrough is different.
I would like more games to have randomized enemy looks, like all the clothing options etc for random citizens in the GTA/Saints Row games. I’m looking at you, valve… Why do all the zombies have identical lab coats and blue jeans on? Mix it up a bit! have like 5 shirt styles, 5 pants styles, and a few extra textures for each that can be recolored, so we aren’t fighting an army of clones. Same with the soldiers… have a few different kits and uniform variants.
I’d like more games to have extra walk run cycles as well. Saints Row 2 let players choose between 20 or so custom walk animations for their player(and iirc 6 different melee fighting styles). Sadly, there was only 1 jog and one sprint animation though. Still, at least the citizens walking down the street had enough of a difference that it didn’t seem like everyone was cloned. It gets tiresome seeing every single enemy move exactly the same way… Spend a few extra days with the MoCap engineers and come up with a dozen or so different walk/run/sprint animations. Especially if the game has zombies with broken/nonfunctional extremities.
I would love to have Runes of Magic’s personal homes and guild castles in World of Warcraft.
I would like to have the old ability from Everquest to put full packs in all the bank slots, not just the ‘backpack’ slots.
Oooh, unrelated but this reminded me of something : in Black & White and an RPG the name of which I can’t recall, there was this very cool interface gimmick : you could cast spells by drawing symbols on the screen with the mouse. *Okami *did it too, but drawing with a thumbstick is way more awkward ;). That said, Okami added a nice little refinement, in that the size of the symbol mattered (for the bomb spell, draw a big bomb and it spawns close, draw a smaller one and it appears further away)
This would make RTSes sooo much easier for me to play. Instead of tabbing to barracks, produce soldier, then go back to wherever : just hold the mouse button and draw a stick figure or something. Instead of having to move the mouse down the screen to click on a unit’s special ability (yeah, sorry, can’t be arsed to remember the keyboard shortcut for all 800 abilities of 400 units 
), just draw a | for the first ability, – for the second, + for the third etc…
Hell, that would also solve a lot of headaches re. pathfinding. Instead of clicking a destination, just draw the path your guys should take. I know you can already sort of do that with shift+click, but drawing the path would be even faster and easier.
That is a brilliant, brilliant idea.
In a similar vein:
I’d like to see terrain effects (slope, water, bunkers, etc.) implemented on a soccer or football field.
It might be worth giving Guilty Gear 2 a rent - it wasn’t quite as frame-tight in the moves and combos as a DMC game, but it had a roster of characters with decent movelists and an interesting RTS/Pikmin-like aspect to the little guys being summoned on your side.
The Baldur’s Gate series with 3.5e rules and the Temple of Elemental Evil engine.
Or ToEE with the storytelling of Planescape. Or any storytelling, really. Fantastic combat and classes, but after the secondtieth level of the Temple of Infinite Sameness, just… no.
Oh God. I’d sell my firstborn for that one.
I’d like to see tackles in soccer. And soccer should drop nets in favor of basketball hoops so scoring would be impossible. It would end the riots after matches.
I’d like at least one football game of every season to be iron man, if they play on offense they must play on defense. I bet guys no ones ever heard of would shine.