I was thinking it might be good to provide a reward/incentive for monthly subs.
What I had in mind was a reward system, similar to Bravery Bonus, for monthly subscriptions.
Basically, if you have a paid subscription for 1 month it does nothing, but as you stay subscribed it would provide increasing benefits at different tiers.
For example, every 3 months of subscribing gives your characters +5%HP/SP or something similar.
Let me know what you think, and thanks for reading!