Keeping older buildings relevant

I would really love to see upgrade options for White Castle, Dark Castle , Witch Tower, Pirate Residence. I mean I love these buildings but they are not worth the building space anymore.

I really would enjoy an option to upgrade them maybe a permanent buff that you could buy during an event to add an x amount to max population of the building.

What about the old watermill and silo?

It's not just a case of suggesting upgrades it needs to be balanced upgrades, for that it would have to apply to all population buildings not just the ones you have named. The main problem is doing it in a way which does not further devalue the value of beans.