Some friends of mine over on 64Digits were tossing around ideas for making this sort of thing happen.
Essentially, you would go on an adventure, by choosing four party members (There were plans to include a choice of up to 12 or so) each with different shottype configurations and abilities.
Controlling them was the standard "tap Z to shoot regularly, hold down for special and to slow down" business with X for bombs and C for swapping out to different members.
Defeating enemies contributed to your experience, which capped at Level 30. I believe defeating midbosses was an automatic +1 level and endbosses were +3, so even the pacifist runners won't be set too far back (They'd just have to put up with waiting out the final boss at level 17)
To avoid adding a randomized player power element to the game, everything was hard calculated given pre-defined base stats, and damage towards the enemies was also fixed (that is to say, that there was no random multiplication in play.)
Discrepancies between players involved HP and HP growth, shot power and growth, range, spread, refire rate, hitbox, grazebox, bomb range and power.
Getting hit subtracted from that player's HP count, and if they were to die completely (Which usually took 2 to 5 hits) then they'd be out for the remainder of the stage.
Item drops served as currency, which you could use to raise bomb caps and give bonus experience to your party between stages.
It was a pretty sweet idea, but actually developing it fell through because it was one in a line of "community" games that expected input from everyone, which, you can guess how well that would have gone
