General idea
I only use miro for my own sake. I never collaborate. The only thing I really ever miss are more editable boards. So I suggest that users on the Free Plan may “unlock” more boards for a monthly cost per board.
Suggested implementation
Each “unlocking” could cost $1/month. So 4 “unlocks” would be $4/month and so on.
“Make editable” can be added as a menu option for each board. Clicking that would give the user the option to setup a monthly subscription to that board.
“Make view only” should then be added as a menu option for each “unlocked” board. Then the user can stop the monthly subscription to the board.
The user should be offered to upgrade their plan if the monthly board subscriptions reach the same cost as a plan that includes at least the same number of currently “unlocked” boards. (At the moment that would be the Starter Plan which has unlimited boards.)