If Bioware/EA Games released the source code I am almost certain that an army of modders will be able to provide a lot of new ideas, angles and posibilities not yet found.
Look at Skyrim. This game is so much alive, and still developing. Bethesda has secured an enormuous crowd keeping the game alive and create new ideas. I have a feeling Bethesda is looking over their shoulders.
I don't know if Skyrim is a very representative example of what you can expect. Yes, it has crazy amount of mods (32k atm on nexus alone) but as counterpoint, look at the Witcher 2. You have people swearing up and down in this very forums how great game it is, it has released a toolkit, and yet the total amount of mods for it on the nexus is around... 300. Yes, less than 1% of what's produced for Skyrim. Even DA2 which didn't provide the toolkit has ~900 mods, 3x that. DAO which did have the toolkit is sitting at ~2.1k
(for what's worth both recent Fallout games have ~12-13k mods each. There just seems to be very
obsess dedicated group of people following whatever Bethesda makes, dating back to the Oblivion craze, I'd guess)