I've been thinking about these ideas for a while as an added bonus for Premium/Premium+ members. I currently am not a Premium/Premium+ member since I do want more incentive to keep my membership...I love CR so I hope you consider these suggestions!
1. Only Events!
I think it'd be neat to cater an event to certain fandoms for any manga/anime you license through Only Events! For those who don't know, Only Events are basically functions based entirely off of one series. For example in September in Japan, there is a Hunter x Hunter only event for the HxH fandom where people get together to trade/sell fan works and do HxH-themed cosplay photoshoots. In addition, people will host little events like talking about headcanons or meeting their favorite fan artists. I think making this exclusive to P/P+ members for popular series (or even long time favorites like Naruto or One Piece) would get people excited since a company would be supporting a fandom by hosting a small event/party. Normally Only Events are fan run.
2. Bilingual Manga
I met a translator who put together a bilingual Japanese/English manga. It got me thinking about how many people who watch anime and read manga also want to learn Japanese. I think it'd be a great incentive for P/P+ members to have bilingual manga so they can practice their skills translating between Japanese and English (or whatever other language). My idea is that each speech bubble in the original Japanese is numbered, and then at the bottom are professional translations (ex: English). This way, readers who want to improve their Japanese skill can read and compare the Japanese to the English translation. Then P/P+ members can discuss the grammar and be active in their learning! Personally if CR had this feature I would 100% retain my P/P+ membership.
Thank you for your time!