I think something that could be nice to add to Tower Defence is you can remove a tower of one of your teammates. Ex.: a tower is not well placed and you would like to place another kind instead. When you right-click the tower the menu to upgrade the tower appears in the left or right corner you could have a block or something else that when you click it something appears in the chat of the owner of the tower and its asking him if he want to delete this tower because someone want to delete it, he could click yes or no. That's it sorry for grammatical mistake English not my first language :).
