RAGE would be pretty cool with a survival mode; however, I think if any MP is implemented it should be nothing like what Doom 4 will offer.

What ideas do you guys have to increase the longevity and population of the game through MP?

Coop missions, maybe add support for 4 player coop, not sure if it's possible because you would have to increase number of enemies on screen at same time.

TF style deathmatch would be great.

Mutant bash tv solo and coop competence leaderboards

This could be a game in itself...compete for accuracy, kills, duration, taken damage, kills per sec...etc.

Could keep on making new challenging maps and scenarios...would be perfect for DLC.

Resources per kill as an itemized and averaged measure would add flavor to ranking.

If anyone knows where damage per ammo type is listed I could get more specific as to comparative values. MP of any form would require tweaks to the engineering formulas or base prices for player parity, or a scarcity of components ie. pop rockets..