Take 3 games...

[Ophiuchus]

They'd have to be pretty similar games, otherwise the combination of the three would end up as a horrible disjointed mess. But then, if they were all pretty similar, there wouldn't be much to gain from combining them in the first place.

...sorry, that's not really in the spirit of things, is it?

Erm, let's go for Super Mario 64 + Beautiful Katamari + Oblivion. I guess you'd roll around Cyrodiil collecting stars to repopulate the galaxy after Mario caused a big Oblivion Gate to open up and swallow all the stars and planets and whatnot. Or some such nonsense.

 

[AlternatePFG]

Fallout 3 + Fallout: New Vegas + Borderlands

A Fallout game with great atmosphere and exploration (Fallout 3), with a great story and characters (New Vegas), and tie it all up with awesome loot and better gunplay (Borderlands).

 

[PUR3_GAM3R33]

Infamous+Saints Row+Warhawk=massive superpowed battles

The customization of Saints Row allowing you to pick any power/customize your persona,the building climbing and karma/upgrade system of Infamous and the wide open battles of Warhawk.

Imagine how awesome it would be for a Geomancer to fight a Photomancer or a Gyrokinetic to fight a Teleporter while a Weapon Manipulator is fighting a Sonokinetic. OMG the possibilities!

ALONG with an awesome story to boot! ^_^

 

[rancher of monsters]

Megaman, N+, and throw in some Bayonetta for fanservice. Super awesome, shooty platformer, with unrealistic proportions for the win!

 

[bl4ckh4wk64]

Ace Combat 5 story/characterization
Mass Effect story/characterization & customization
and I'm stuck on the third, but those already are two completely different games.

 

[baddude1337]

A few:

Ghostbusters: The Game, Evil Genius, Dragon Age Origins:

I would love a game that had gameplay like the Ghostbusters game did, but also a kind of management / free roam style, where we build our own Ghostbusters franchise around the world, take on the missions we want and generally just do what we want. That would be so awesome.

Marvel Ultimate Alliance / Infamous / Oblivion:

I would love a free roaming game set in the Marvel Universe that let us create our own/ use an existing character, with as much to do as Oblivion and gameplay like Infamous.

Total War Series, Star Wars: Republic Commando or Battlefront, Command and Conquer Series:

I would love a galactic Star Wars strategy game with battles that had as many units as the Total War games do in a battle, but also let you zoom in and control a soldier, that could play similarly to Republic Commando or Battlefront.

 

[The_ModeRazor]

Hmmm...
Planescape Torment, Saints Row 2, SW Battlefront 2.

*universe implodes*

 

[Kohake]

Super smash bros melee + Guilty gear.

That'd be it. I guess you could add another game for theme or story, but with the depth and creativity of Melee's system and the balance and awesoem that is GG, I can't think of a figting game I'de rather play.

 

[Assassin Xaero]

STALKER (gameplay mechanics and atmosphere) + BioShock (world + artisticness of it) + Amnesia (added atmosphere and horror)

 

[LobsterFeng]

Batman: Arkham Asylum+Fallout 3+Minecraft?

 

[Crono Maniac]

Gameplay as good as Final Fantasy IV's, characters at the level of Final Fantasy VI, and a setting as unique and thematically rich as Final Fantasy VII's (particularly Midgar.)

Chrono Trigger has gameplay 'almost' as good as FFIV's (better in a lot of respects, but the dungeon crawling is a lot less engaging), characters 'almost' as good as FFVI's, and settings 'almost' as good as FFVII's, and it's my favorite game ever. Ergo, the greatest game ever would be as good at all three. If it ever gets made, my bet is on Atlus being behind it, since it certainly won't come from Squeenix.

 

[ragawolf]

metro 2033, super smash bros brawl, and minecraft

 

[-Drifter-]

Minecraft+Thief+Darkworld = Thieves of Darkcraft.

Thieves of Darkcraft would be an RPG set in a huge, randomized fantasy world based off of olden days mythology and a focus on realism, with Thief's sword play and stealth mechanics. Given the huge, open, random nature of the game, the story wouldn't require you to do it in any particular order and would probably be told mostly through exposition and old texts. Somewhat similar to Mount and Blade, I suppose, but with an actual end goal and focused mostly on the player character and maybe a small party of team-mates.

 

[PatSilverFox]

inb4 gears of halo theft auto 5

Hopefully no one posted that.

 

[-Drifter-]

Batman: Arkham Asylum+Fallout 3+Minecraft?

You play as a martial arts master with access to futuristic gadgets in a open world post-apocalyptic war zone that is randomized during each play-through, and chock-full of destructible scenery. Occasionally (usually at night) you are set upon by mutants who explode themselves when they get close to you.

 

[Arrogancy]

Bioshock+Bioshock 2+Assassin's Creed 2. This combination only works if you take the heart, spleen, intestines, and brain of Bioshock, add the muscle structue of Bioshock 2, and the voice and the face of Assassin's 2. To put it another way the core story, characters, and plot of Bioshock, add the combat system and some enemies from Bioshock 2, and the character arc and awesomeness of ending reminiscent of Assassin's Creed 2.

 

[CrashBang]

Zone Of The Enders 2 + Arkham Asylum + Final Fantasy IX = OMGBESTGAMEEVAR!
Seriously, I think this would be my absolute perfect game! A steampunk, classic fantasy open-world stealth/fighting game with mechs. Oh god, I don't know how to handle this level of amazing

 

[Riddle78]

Dungeon Keeper + Infiniminer + Dwarf Fortress = Minecraft.

 

[Orcus The Ultimate]

Beyond Good & Evil + Psychonauts + Mcgee's Alice

or

Deus Ex + XCOM + (any of those above)

i wonder what it could turn out to be...

 

[cgentero]

Metal Gear Solid 3 + Ico + Resident Evil 4

Outdoor environments, Stealth\Evasion focus, Large scale platforming, Long periods of escorting, Survival elements, Semi fixed camera(with the ability to pan, zoom, and switch angles)