How about instead of buying back towers, you can pay gold to repair them when they are out of battle, and the repair stops either when done with how much repairing you paid for or if the turret gets into battle.
But if it is destroyed it is destroyed.
Also, another option would be worker minions either spawn after x number of waves or you have to pay gold then a wave of them spawns.(not sure which would be better)
Then they have to walk to the turret and repair it or rebuild it. And they can't protect themselves from enemies.
It would be nice for the ability to buy upgrades for turrets, or for turrets to gain exp and level.
Though I don't think they should ever have a range increase. The upgrade options that I think would work: attack damage increase, fire-rate increase, hp, armor, and magic resist.
Also if there was a turret upgrade system, I would love to see the ability to have auras around turrets that either debuff nearby enemies, or buff nearby allies.
But I am not sure if upgradable turrets would make games last for hours straight.(which would be extremely annoying.)