While Resident Evil 6 has only just been announced, eagle-eyed fans are scouring the Internet for any new bits of information they can find. A product listing on Xbox.com confirms that co-op will be returning in Resident Evil 6--but it may be much more ambitious than what Resident Evil 5 offered. The "Features" section lists support for two player offline co-op, but another bullet point features six player online co-op.
In addition, it appears Resident Evil 6 will feature some other online multiplayer component, with support for up to eight players. Resident Evil 5 later offered a "Versus" mode via DLC, which allowed four players to go against each other in deathmatch combat.
Apparently, Resident Evil 6 has been in development for two years--which makes sense given it's been about two years since the release of Resident Evil 5: Gold Edition. A demo is being promised, but it won't be available until closer to release.
The return of zombies for Resident Evil 6 also promises the return of "survival horror elements... mixed in with classic Resident Evil moments." So yes, you may be jumping atop of helicopters and sliding into cover... but it won't be entirely action-packed.
Resident Evil 6 will be available on Xbox 360 and PS3 in November; a PC version will follow afterwards.