So, please, be gentle. I’m spitballing here…
Pay to OP
(Assumes the software can do this, meaning assumes facts not in evidence.)
Registration is free. You can respond to your 
’s content, a free registration allows you to create up to 3 topics.
Once you use your free topics, in order to create more, you pay a small fee. $2/mth. $5/yr. Whatever.
(And as I’m typing this, I am coming up with the same objections you are, but in a session like this, the goal is to get the ideas on the table, not say ‘THIS is what we MUST do’.)
So, I’m thinking membership tiers:
Charter Members - keep those for us who have the title, remove the charge, unless the CM opts for the…
Contributor plan. Free (except for CM’s, who still pay their current CM charge), this gives 3 OP’s and PM access (limited? Ideas?). No avatars, no signatures.
Creator plan. Contributor plan + 15 OP’s, unlimited PM’s, $5/year
(You can buy more OP’s if you want to, I guess. $5 for 15 more topics, $10 for 40, why not?)
Doper plan. $5-10/mth, unlimited OP’s, unlimited PM’s, Avatars, signatures, the ability to imbed images (can that be limited by user?), the whole gamut of functionality.
Again, just spitballing here. I don’t care if the middle tier plan has more or fewer OP’s, or if Avatars are tier 1, 2, or 3, or whatever. I don’t even know if the OP idea can even work. ¯_(ツ)_/¯
But the SDMB shouldn’t limit itself to one plan. You don’t want to lose new users (hence giving them 3 OP’s, given that many people register here specifically to ask a question), and you don’t want to lose the current members, and you want to give new members a reason to level up on the tiers.
Tiered pricing, which has the above as the goal, is going to be the best way to do this.