Wouldn't Sonic be too fast for Mario?
My ideas over the years are mostly sequels (heck, there can't be many good ideas left)
-20th Century: Total War - 100 years with a modified total war engine, with separate in-depth WW2 campaigns covering the European/African and Pacific Theatres.
-Mongol: Total War - two campaign maps, one beginning from Genghis Khan unifying Mongolia to his death (Central/East Asia + Japan), including nations like the Mongols, Liao/Khitai, Xi Xia, Song China, Japan, Khwarezm, etc. The other would be west asia/east europe and include the Golden Horde, Il-Khanate, Byzantine Empire, Crusader Kingdoms, Hungary, Rus, German Empire, Egypt, etc.
-Three Kingdoms: Total War - self-explanatory
-Star Wars: Fleet Battle - think Battlefront meets X-Wing series. 100 a side (easily possible, the PS3 game MAG has 128 a side), controllable capital ships and fighters, boarding, etc. Should cover both Prequel and Original eras.
-Ratchet and Jak - Ratchet crash lands on Jak's planet, helps to avert crisis. In turn, Jak accompanies Ratchet to avert another crisis. Local and online co-op play.
-Mirrors Edge 2 - ok this is being made. Please give us freeroaming, DICE.
-Battlefield 3 - actually this is being made under the guise of Medal of Honour - but recapture what was good about BF2 (teamwork, vehicles and classes) and balance it this time
-Dynasty Warriors 7/8 - proper online battles, more involved combat system, real AI, customisable characters, and make it use the
playstation motion controller (see bottom for video demo)
-Star Wars: Jedi Knight: Clone Wars - first person shotoer/lightsaber, using motion controller above. Multiplayer duels, massive battles vs AI
-COD Modern Warfare 3 - Russians retreat from America, massive battle in New York including Empire state building, Ramirez to get crushed by statue of liberty destroyed by Russians. Roach survives, he Soap and Price fugititves hunting Makharov and clearing their names. Co-op campaign!