timeline is really tough on this one… we feel pretty strongly at the moment that 5.0 will not release without four main features… plugin api, templating, stronger buddypress integration, and increased performance through caching or other means…
I feel pretty safe in saying that it wont be before the end of the year. The changes are going to be huge and will require a significant alpha and beta period… additionally, we want to give some time for aspiring plugin authors and themers to play with the apis and provide some feedback…
when we get closer to the theming and plugin api (I almost complete converting the pm system to plugin for example) being real, we will likely open up a 5.0 specific forum for discussions and testing… As a two man dev team, however, there is a limit to how much we can do – how fast…
As to ideas, we already have a forum set up for ideas… and we have a bug tracker page (see nav menu) where you can follow along… additionally, 5.0 is already in development and available via svn, though it is FAR from stable at this point…