If you could change one thing...

[ThePantomimeThief]

Star Wars: JKJA - I'd love to see a way where you can kill Rosh Penin, but join the cultists and the Reborn, instead of going completely Sith and killing everything you see.

yeah that would be sweet, I always wanted to team up with them and fight alongside the reborn instead of just killing everyone.

MW2 would have been much more fun if you could take guests online with you a la Halo 3. Or how about a system link including split screen?

The driving sections of Mass Effect and Borderlands could have been made easier and put flaws in games that otherwise I think are brilliant.

 

[Dark2003]

Final Fantasy 13, i would make the game play similar to crisis core, a free roaming battle area, this would also open the game up to co-op as well

Devil May Cry 4, kill nero's girlfriend and make it about revenge, and also get rid of the character switch midway through it and make it one game one story different paths taken

 

[Schwerganoik]

Fighting games in general: Add a blocking animation, instead of crossing arms or holding out your weapon becoming a magic shield.

Oblivion(Damn, only one thing?): Keep rats and goblins at level (instead of magically becoming level 50) and have higher-level quests send you to places with higher-level creatures.

All games with unkskipable cutscenes: Make cutscenes skippable. Even if they're awesome, I'm not likely to enjoy them the second or third time through.

Dungeon Siege: Make item sets more powerful.