yes it is entirely possible to make a game of this scale but probably unlikely to happen. Creating a game from scratch like this is a huge undertaking and given that the people working on it would be hobbyists, they wouldn't have 40+ hours per week to work on it. By time it was complete it would be obsolete
and don't forget that the majority of games are not open source, so if you are talking about porting one to linux, it would be just a little difficult to do without the source code
and If you are talking about taking a game's engine and creating a MOD for it, this is quite possible and is done quite freqently (look at counterstrike).
But if the engine isn't written for linux then you can't make a mod for linux.