Whoever gets the code should first try to get a nice team of people you can turst to start listing all the bugs that should be fixed in JJ2 (so leave out the wallclimbing and MCE bugs as fixing them will mean a lot of levels will not work anymore) and a list of things to add. Someone, preferably someone who is easy to contact with much time and who doesn't get stressed very easily, should then keep all information up-to-date on a location that is accessable to at least all of the team members, but preferably the whole community as it shouldn't contain any secrets anyway. Obviously, a public 1.25 category should be added to the forum with progress reports, to-do lists and threads were everyone can post ideas or the game. Ideas should best be categorized into seperate threads, think of "Bugfixes" "New evets" "Implemented gamestyles" "Game options" etc. Maybe you could make two threads for each type, one for completely worked out features and one for vague ideas.

Oh, and don't forget about an IRC channel ;p