Here's hoping Teemo gets a rework with his new skin
Teemo's in a not-so-good spot right now and while I don't profess to have answers to fix his kit, I do think his Mushroom needs help.
It's invisible. The counter to this: Ward for it, use lightbringer, use a few different items that can expose it. Kill them. The problem with this to me is that his shrooms are either not impactful enough to warrant countering, or if he's ahead, these things are hard counters to his ult and make him near-useless.
Proposal:
Make his shrooms visible with no way to interact with them (other than triggering them) and lowering damage.
The counter: Walk around. You can see them for free now. However you cannot kill them. Or, you can send your tank with Merc Treads in to intentionally trip them and clear them.
What does this do for Teemo? Gives him visible "slow mines" rather than invisible "damage mines". It gives Teemo a more solid web of safety if he's willing to take the time to weave it. They are also easily outplayed (go around, dash over/through, send tank in), so a unique, powerful ability with obvious counters but still having strength
Okay I'm done lol. I'll think on his other abilities, but I feel like this is a solid suggestion for Teemo. No longer would he have hard counters, and he could reliably use his ult knowing it will have an impact.