Multi-Game Online play

Not sure if this has been done before, talked about before or even mentioned?

How hard would it be for developers to make multiple games that share the same online environment/server etc.

I'm not talking totally different genres, even different games but for instance Resistance Fall of Man - the sequel to be made Resistance Rise of man - how good would it be if you could only have one of the games but play online against someone with the other game. Simply download the relevant map to the PS3's HDD and the opposite games player models and perhaps any new weapons etc - would/could this work or would the download/patch whatever they want to call it have to be to large that it wouldnt make it worthwile.

Could this than open the door for say a cross game type online experience where we could see say Rainbow Six players vs Ghost Recon in a nuetrual environment? Each type of player having its own pros and cons.

This idea could defintly prolong the life of games, even if they charged a small fee to download the required update/patch.

Thoughts please?

Well I know myself that if the Resistance sequel could play online again people from the first game.  I would buy Resistance used and rent Resistance 2, and they would lose a sale of Resistance 2.

Maybe thats just where my head is at. 

Thats what I was thinking - YOU can only have access to the weapons from your purchased game, but for the sake of the game working it would need the weapons physics/damage/reactions etc downloaded to your machine.

 dont think it would impact sales all that much - its not like u are getting anything new to the original, just new and more people to fight against with different styles and weapons to fight against.....

Be like getting an expansion with new enemies to play against not much changes to your game play just new opponents.