Personally I think, rather than the scaling, the base damage, cooldown, mana cost, and most importantly, duration should all be cut. They currently have a ten minute duration, which is a hella long fucking time in this game. The other changes would be to compensate for significantly reducing the duration- in return for not lasting as long, he's allowed to place more. In short, this would force the Teemo player to be more active about setting up shrooms and maintaining them, rather than dropping a shroom in a distant lane/jungle and letting it be able to sit around for half of some games while he forgets about it.
The drawback to that is it lets him set up zone control of lane quicker/better, hence the reduction on base damage.