I would suggest moving some of his power budget around. Maybe a higher cooldown on the blind and/or reduced mushroom damage in exchange for greatly increased on-hit and poison damage from Toxic Shot. Taking both nerfs would allow us to increase the vision radius of the mushrooms.
Both the reason Teemo is so frustrating as well as the reason he's not viable is because of his ultimate. His mushrooms are frustrating, but since they're so much of his power budget and pros know how to play against it, he's not seen in pro play. By swapping some of his power to Toxic Shot and reducing damage from the mushrooms, it shifts his build into a Runaan's based AP/AS build with items like
and
. Now, 3-item Teemo is a force to be reckoned with, and he's a late game hypercarry who can take over games a la GP (of note, Teemo is a great matchup into GP).