So this idea is a simple and to be honest has buged me since game one, finding a way to spend all those stars that you build up in the games and have nothing to spend them on. here is my list of ideas and feel free to expand it if you have any more. (Side note: they could add these as another part of the gnomes shop.)
1:buy a perminant upgrade to get a 35% discount on all items in the shop this way we can still get all the upgrades and be able to stack up on items faster. (This unlocks after you squire the amount to purchase all the upgrades and the discount so you don't scree yourself over.)
2: use stars to unlock concept /development art.
3: we use the stars to unlock a new challenge in the game like and endless level or a treasure trove that allows us to play sandbox on endless where we fully stalk construct all the towers and just let the havoc break loose, (this would lock our items from being used, though).
4: have a hidden journal section in the encyclopedia about the kingdom lore and we use the stars to unlock the pages. (Complete with colored depictions)
5: use the stars to unlock some mini games that help us earn gems (pizza rush?)
6:use the stars to unlock a crossover level where you get to construct using all the towers from all the games.
7: use the stars to unlock some funny modifications to reinforcements or towers.
Well that's all I could think of at this point. love it, hate it, expand it, or discard it. Whatever the case, have a nice day.