Heya!
Thanks for the feedback! Unfortunately, this isn't something the team has plans to add.
1820 skins would mean giving him a totally new VO script, which is absolutely not in the scope for this particular skin. You're correct that 1350 RP skins sometimes have new processing on their voices, but this isn't something Little Devil Teemo will have.
Adding processing to a skin means hand editing each line, as opposed to just sticking a filter on the skin. Due to the timeline of the skin needing to ship (we want to make sure these skins are out by Halloween this year! ;]), this isn't something that's feasible at this point in development.
For a bit more context here - when we make changes, it means testing a good chunk of the skin again, which can take a bit of time! When we make a new skin, we're also testing every other skin a champion has (and Teemo has a ton of skins!). We do this because we want to make sure a new skin didn't break any of the older skins. This can happen, and has in the past! Most recently, the Battle Bunny Riven chromas broke the Q sound on BB Riven (and its chromas).
We are always open to feedback, and more minor changes are things we can accommodate, so I don't want you to feel like we always say no, or that your feedback isn't being heard, because it certainly is! ^-^