interested, yes. I have been thinking about this type of thing. Basically my plan is to have an extra usergroup "subsriber". A subscriber would probably pay something like $20 per year or something like that and would get (things i can think of)

i) large storage space for pms.
ii) custom avatars.
iii) an extra profile field showing they are a subscriber and this is displayed on postibt.
iv) removal of any limits on posts per page or threads per page
v) possibly a custom style set that does not have banner ads in it (this might make it more expensive so a 3rd layer.

and whatever else i can think of.

now they way to do it would be to set up a different user group, subscribers or something like that.

i) can be done, there is already a differentiation made between admins and general users so would not be hard.
ii) a bit harder but i doubt much of a problem
iii) easish to do
iv) not sure best way but possible of course
v) this is realtivly easy dependant upon how you use style sets. If you only use them for say co-branding then you could just set a user's style set to the one without the banner banner. I use style sets for forum control (design) so couldn't do this so would have to find another way. It would be easy if template code was parsed so that you only load a template if the user is != "subscriber" but you can't. Sure there is another way round.

I am not saying i am going to do this, waiting until final to start anything big but there is a possibility here. It is a nice idea of trying to get people to pay something but getting some real benefit for it. I am hoping someone will sometime make an email --> pm hack so you can use the pm system as an email account. This would make i) a lot better especially if you could allow pop3 and smtp mail using it on a selective basis (possible if you use qmail i would think?).

Anyway, some thoughts and yes something i am interested in. What does everyone else think?