I think it's pretty clear he has to be a Yordle and very probably a Ranger. I don't think he should be able to have both the mushrooms AND the blind. That's two abilities and TFT has room for only 1. Unless you do like the Robot and Exile origins and give him a unique extra origin with one of the two abilities, and put the other into the actual kit. But that would be unimaginative and kinda OP.
Here's my take on TFT Teemo:
Idea 1 [Basic skin]: His ability is somewhat spammable and consists in throwing shrooms at empty tiles near low health enemies that explode when stepped on. They poison enemies, damaging them over time. Then Teemo will attack the poisoned enemy with the lowest health and gain a big ammount of attack speed until the poison wears off. His basic attacks against poisoned enemies cannot miss or be dodged. Origin: Yordle. Class: Ranger. Tier 4/5.
Idea 2 [Omega Squad skin]: Basic attacks deal bonus spell damage that can also crit. Origin: Yordle. Classes: Ranger and Assasin. Tier 3.
Idea 3 [Demon skin]: The same shrooms from Idea 1, but instead of a burst of attack speed and a switch of target, they briefly disarm, deal more damage and cost more mana. Origin: Demon and Yordle. Class: Sorcerer. Tier 4.
I also like Teemo a lot and would love to see him get added to the TFT roster. RITO PLZ