Or Shang Tsung from MK1 vs. Shang Tsung from (Well, MK:DA probably wouldn't work so well, 2d vs 3d).
That's already been done... remember Mortal Kombat Trilogy?
They had all the fighters from all the various Mortal Kombatii, in the MK4 engine, right? But did they have the different versions from the various incarnations of the game? i.e. the "old guy" Shang Tsung from MK 1, with his moveset and fatalities, and the "young" Shang Tsung from MKII?
Plenty of fighters have done "compliation" versions with all of the characters from all of the games(Tekken Tag, MK Trilogy, MK Gold), but this Street Fighter version has each incarnation of each fighter from each game in the series, along with the improvements to the engine and movelist. From the Press Release:
Concoct such matches as Ryu from the original Street Fighter II as he uses a rapid light kick while a second Ryu counters with a Super Combo from Street Fighter II Turbo