My 1 cent thoughts :
google ads if needs to be. It is extremly pleasent to have a forum without any adds. Maybe 2 versions of the forum (with/without ads); pay for without.
It would be interesting to see how many users (servers) are out there using froxlor. This would give an idea how much could be raised.
pay for features, good idea.
sponsers, yes. Is this option currently available ?