It's great to have a private section of my website for paid members, but the following would enable me to grow my membership and make the experience smoother for current members - In order of priority:
1. Discount codes for paid plans - I want to be able able to run a time-bound special, offer discounts to specific audiences (like podcast listeners but not all site visitors), etc. The ability to create discount codes/coupons like I can for the Wix store would enable me to do so.
2. Allow members to manage their own paid plan account - right now, if someone wants to upgrade, downgrade or cancel, its a big to-do. They have to email me, I have to cancel their purchase and then they have to re-purchase a new membership at the new level.
3. Keep Plans Active but hidden from the published site - It would be ideal to be able to change my prices while keeping old subscribers at their current price. This could be done via a coupon code or by allowing me to "hide" plans from the live site but still have them continue to charge customers. That way I can 'show' only the new plans for any new subscribers.