I had a glance through the thread titles and didn’t see anything similar to this idea but I’d be surprised if this hasn’t been suggested before.
I think it would be a good idea to have an “upgrade donation box” in keeps and towers. A way that it could work is that people donate silver towards an upgrade and when the necessary amount is reached that upgrade is purchased. This way everyone can contribute regardless of individual wealth and not have to depend a single benefactor.
There could also be a list of player names and the amount donated by each individual to help build community.