It's a decent idea but we'd run into the same problem we have with the wards. What happens where there is no public space left to rent, and some players are renting the space but not using it under the pretext of "I want to make sure I'll have access if I decide I want it later"?
Much better if they had moved gardening into Island Sanctuary altogether, though I guess that's not happening since YoshiP is insistent the content has no competitive aspect and the MB is one of the few competitive features in the game.