Make Teemo's shrooms turn off if he's away too long.
Basically, the same logic that led to forcing Heimer to stay near his turrets also applies to Teemo - champions have no business being able to keep waves pushed from the other side of the map without paying some kind of cost. And in the same way Heimer's turrets cost him basically nothing to keep out all the time, Teemo can just throw shrooms without a care.
This would also help rein in Teemo being one of the absolute least fun champions to play against because of how trivially he can cover the map in highly damaging traps that also pack potent CC, a relative large AoE, etc.
I'm not saying that they need to have as small a radius as Heimer, or turn off quite as fast. But if Teemo returns to base and then goes bottom, why the fuck does he get to defend top turret while doing so?