I have a suggestion to help the transition to 5* without changing things drastically. It is very similar to the new(ish) 4* vaults they have been using.
Once a week, have a Legendary token that costs 30 cp that features 1 specific 5*, similar to when a new 5* character is released.
So one Thursday, for 1 day only, have a legendary token that costs 30cp with a 10% chance for Silver Surfer and 90% for a 4*. Next Thursday have a 10% chance for OML and so on.
It would help the 4* to 5* transitioners get the covers they want a little easier, without making it too easy. The mechanic is already in use for new 5*, so the coding will be easy to implement, and the increased cost per token will offset the higher chance to get a specific 5*, while not giving too many as it still runs on rng.