I always wanted something to enforce tower defence! The only thing available in dota is roofs living armour, that both increases the towers armour and its regeneration. So, I dont have any great idea but if its a suggestion worth checking out maybe you guys could help.
The use of the item would be in lanes where you get pushed really hard, and in need of keeping the tower up for a little longer.
Maybe an item that regenerates the tower with 100hp. (say 2-300 gold)
Maybe an item that gives the tower some +armour for a set period of time.