I suggest you make his turrets incredibly easy to kill and give them a short cooldown/low mana cost. This way Heimerdinger can drop turrets around as a sort of "damage over time spell" that scales with how many he can create. This way Heimerdinger doesn't have to play so defensively, he can fight like other heros because he isn't so invested in his turrets from using up so much time and mana. This also placates people who say the turrets are annoying and extend the game length. Anyone will be able to mow the turrets over, they will mostly be used to push a lane via killing minions. As a bonus, people will be able to play him defensively/offensively this way; it opens up a lot of possiblities. Then make his ultimate have an incredibly long cool down/high mana cost and give those turrets high stats. This way he can set up useful traps/hero deterants. The nicest part about this fix is that it requires very little code change, just a few number swaps. It also nicely differentiates between his turret spell and ultimate in a way that makes him useful throughout the game =)

This is more or less what I envisioned. The yellow turrets would be very expendable and only dangerous to minions; the green would be much better at killing minions but would have to be protected by heim until they get updraged to red; the red turrets would be quite deadly.

this isnt a codeless change, you would have to make changes to the code to implement this change.

I don't think so. The turret creation spell's purpose would simply shift from a defensive-fortification spell to a damage over time spell. Their purpose would be to kill minions; being easily killed via splash damage is exactly what we want to be possible. The spell should have low cooldown, low mana cost, and produce turrets that a player isn't overally invested in. The Upgrade turret spell would then turn these weak turrets into permanent turrets for defensive purposes. The fully upgraded turrets would ideally be very difficult to kill and 4-6 should do damage equivalent to other hero's ultimates over a reasonable period of time.

erase green turrets, make red turrets more badass, increase ultimate manacost and cooldown, turrets explode upon death dealing magical damage (+ap scaling). Now that's a champ I would play