I think it would be nice to have some way of seeing which kingdom buildings have upgrades available without having to click at every single one of them.
Currently, clicking on the box at the bottom of the kingdom screen displays the names of all buildings in tiny black boxes.
My suggestion would be to add some information about available upgrades to these boxes.
E.g. they could be green when an upgrade is available and I can afford it, yellow if one is available and I can't afford it and black if none is available.