A "Buy Max" setting for upgrades would be nice. Or a selection like other idle games. (Buy 1, 5, 10, 25, 100, Max)
Also upgrades for things like Building regeneration (wall Regen only?) and upgrades for the speed at which they regen, and tower range (unless that's a building unlocked later).
Also, need more walls + buildings, the empty space annoys my inner perfectionist