Gamespy has an interesting article,
10 Theoretical Video-game Crossovers. I wonder how many have already been fanfic'd to death already.
10. Virtua Fighter vs. Tekken - I'm more of a Soul Calibur guy but knowing there's a plethora of characters in both gameworlds, the possibilities are endless!
9. Tomb Raider of Persia - The Prince meets Lara Croft? Maybe that would have made a better plot for the first movie (wasn't there some brief time-travel?)
8. X-Wing vs. Wing Commander - Possibly two of the greatest space shooters ever. Although everyone swears by X-Wing vs Tie Fighter, I've never played it and I'm more of a fan of the Freespace series. That being said, Freespace vs. Wing Commanders! TCS Ticonderoga against the Shivans!
7. MechWarrior vs. Armored Core - Meh.. sure..
6. Unreal Quake Tournament - I love UT but I never found it had much backstory. Perhaps the marines of Quake find the Strogg are secret backers of the galactic fighting league?
5. Half-Life: Halo - This has got to have been done already in the fanfic-realm. As the article states, Master Chief's got the brawn and Gordon Freeman's got the brains. The only thing better would be
Metroid vs. Halo... 4. Devil of War - Kratos and Dante. That's the most interesting team-up in the list, personally. There'd be so much animosity and character interaction between the two, someone has probably already turned it into slash.
3. Mega Metroid - Rockman teams up with Samus. Uh,
dot_chan?
2. Command & Conquer: Azeroth - The lamest entry in the list. Sure they play in both the same genre but how would you mash up these two together? Kane decides the GDI is too strong so he opens a dark portal to Azeroth and ally's with the Horde? Selling Tier2 Tiberium set, plsmsg kthxbye!
1. Metal Gear: Splinter Cell - I think it would be interesting to pit Sam Fisher against Solid Snake... I've never played Splinter Cell.