I don't know what your total plans are on this game, but I think it has a lot potential to grow big. For upgrades on the masklings and the attacking system I would like to see:
-Barracks to train soldiers (Like i posted in the other comment section. I think this would improve the game a lot because now, the only defense mechanism are the towers.
Building further on this idea. I would implement the ability to attack/take over maskling camps. So a second settlement can be build like that. For this, the masklings need to build a city with the elements you have to build your city with. But I think this would make the game more entertaining.
-Implementing big houses/appartments. In my game now, A wave of 300 immigrants strenghtened my city. So I have to build 300 houses which is a ton of work.
In addition to this, you can add a function where you hold shift en move your mouse where you want to place a series of buildings.
-A fast forward button would be handy when you just have to wait.
Hope I don't irritate you with all my enthousiasm about this game :)