for the future it would be good to have more maps but without objects so players can create their own maze without things in between and maybe bigger maps
edit1: normally on other games when upgrading turrets/anything u don't have to choose a card in a game to get that upgrade, having to choose the card to get the upgrade makes the player think twice before upgrading anything because maybe u dont get that card on your game which can be bothersome/annoying for the player, maybe change some upgreades to be already in place when starting a game e.x. placing more turrets, X turret having an aditional effect
edit2: Endless mode, more rounds instead of the current max 50 and/or more difficulty