First of all, Talli is right. Call it 5.1 ;)
Okay, three months release cycles sound nice, but we'd have to see how it works out in practise, as the release managers have other projects (paid for) to attend as well. On the other hand, it would make it easier for people to say, yeah, it's okay, I will wait til the next version to put my code in, it's only three month away.
Concerning the Version 6.0. I guess we should branch a version 6.0 if we want to make changes again, that affect the whole system (like internationlization and noquote have done).
Other than that, let's go for it, branch version 5.1 on 1st of January with a focus to fix all outstanding bugs by first of February for a release. Would be great if this actually happens ;).
Concerning the scope you mention, don't overstretch yourself and get too ambitious. I think work-flow enhancements would be nice, a better bug-tracker if funding is there, as well (we are using it for AIESEC bug tracking with ton's of users posting bugs, requests and tasks, so we are more than willing to hand over improvements and ideas). But CMS or Project Manager would be too far a stretch in my opinion.
