Tower upgrades?
What if each tower could be upgraded at some point in the game? (Maybe after some sort of quest, like "farm 50 minions in this lane", you could get the option to upgrade the most external tower of that lane)
Possible upgrades could be, for example:
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CC upgrade: the tower now shoots a laser ray that deals less dps but acts like a tether and Grounds the target. (good vs dives)
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Splitpush upgrade: each wave spawned for that lane contains 1 (or maybe 2?) extra allied minions. (good to apply map pressure)
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Bastion upgrade: the tower is more fragile but standing close to it gives bonus HP regeneration. (good vs poke)