But you can not control features available to them. If you can not able make it attractable it will be unsuccessful business approach from the beginning. We can't do that by all our selves.
All of big forum in world wide are having high bandwidth problems, dedicated server fees and other things that will stops forum progress. If you able to give a feature from you forum software I believe all of these forum owners would think transferring their forum to yours.
Small fees with quantity will make difference. I know a site that called
www.pocketpcthoughts.com making this kind of thing internally coded. Now you can see in their forum every hot topic has one or two paid users.
What feature could they have ;
A cool icon under their avatar.
More pm limit
More upload limit
More customizations
Access to mobile forum
for very low bandwidth (maybe only text)
and low pixel compatible
Query prioritizations (if could be) or private server
Access to special forums
Email blast for subscripted forums
Able to reply from email
What do you think ?