I picked up Call of Duty: World at War yesterday. After only an hour or so of playing, I joined a friend in the multiplayer mode that involves fighting zombies from a boarded up house. There are a number of things that make it a lot of fun with friends, but one element in particular has me wondering if it could be applied to campaign gameplay.
The player gains points for killing zombies and repairing defenses. Headshots and melee kills award more points than regular kills. You spend these points on weapons during gameplay.
It struck me as a relatively novel system. First, you're purchasing aid while enemies are still coming at you, during brief moments of respite between attacks. Unlike in Bioshock or in other games, events are not paused as you purchase help. And second, you're earning the credits for these purchases constantly and automatically (not in the form of pick-ups or something similar). Combined, this makes for a fun and unique experience.
I'd love to see a similar system used in single-player or co-op campaigns. It could make a game wonderfully dynamic.