Added 45 minutes later
I'm not sure if this is the right approach. I tried it on a few standard maps. Leprechauns, Fairy Rings, or Pink Dragons simply don't appear on them. The only thing that's somewhat common and not too valuable is the Fountain of Luck, but
first, it's only one tile, and the new building should be the same size as the old one,
second, it's probably too common,
third, as far as I remember, banks on the map aren't guarded by anyone because there's already a guard inside, and next to this fountain, there's usually some creature guarding it.
So, I'll probably have to abandon this idea.
Instead, I want to replace the Gargoyle Bank with something more versatile, like a "Bandit Camp," where I can assign some weak and medium creatures as guards. There are already enough banks with creatures from the Academy; there's the Mage Guild in the Pyramid and the Sunken Temple, which by default have Titans and Golems.