This would be an interesting mechanic, I think. One that would give more value and worth to a level 20 Main Building at ALL stages in the game.
Put simply: The level of the Main Building would also be the maximum level possible of all other buildings and fields within the village. If your MB is level 15, for example, no other building can be upgraded beyond level 15 until the MB itself is upgraded first.
As a result of this, it may also make it too easy for enemies to gimp the village into being unable to upgrade anything if the MB is constantly being catapulted to zero. this is why I think if such a feature went through, the MB should also become untargetable like stonemason, crannies, and trapper buildings.
This would also get people into upgrading their Main Building sooner instead of forgetting about it because it doesn't really give such large benefits until other buildings are reaching mid-high levels anyway.