Concerning bosses, I personally think they could be viable. They'd just be Megaman-esque boss things that spawn (along with their boss room) when you reach certain point values. During the fight, paint would have to be supplied to the player somehow, and the screen would temporarily stop scrolling, and after the fight, the screen would continue scrolling, and play would continue like normal.
Regarding spawning and such (for both the levels themselves and the enemies/items in them), I was thinking of setting up a simplified version of the AI Director that they have in L4D. It spawns new areas (usually spawning a whole screen's worth at a time just offscreen) according to a few values, one of which being a difficulty meter. As the game continues, this meter slowly climbs, which results in harder areas being created. This "Director" would also spawn enemies and paint cans as determined by the difficulty meter. To toss things up a bit, perhaps the Director could occasionally declare a "PANIC" mode, in which the scrolling speed drastically speeds up for a short time.
EDIT: Though, this "Director" would probably have about as much "intelligence" as a bug.
Regarding spawning and such (for both the levels themselves and the enemies/items in them), I was thinking of setting up a simplified version of the AI Director that they have in L4D. It spawns new areas (usually spawning a whole screen's worth at a time just offscreen) according to a few values, one of which being a difficulty meter. As the game continues, this meter slowly climbs, which results in harder areas being created. This "Director" would also spawn enemies and paint cans as determined by the difficulty meter. To toss things up a bit, perhaps the Director could occasionally declare a "PANIC" mode, in which the scrolling speed drastically speeds up for a short time.
EDIT: Though, this "Director" would probably have about as much "intelligence" as a bug.