Teemo: Only Extremely Mana Dependent Mage That Didn't Get Buffed in Patch 4.20
This is literally the best short term fix. Give him the mana regen rate of the mages that got buffed in patch 4.20. That will allow him to synergize more with mana regen items. Allowing his utility based builds to feel smoother and allow a larger variety of Teemo builds. Currently, Teemo can only build extremely high damage builds because he synergizes extremely poorly with CDR.
The following mages / tank supports are being changed to have 6 base mana regeneration (mp5) and +0.8 mana regeneration (mp5) growth stat: Ahri, Alistar, Anivia, Annie, Azir, Blitzcrank, Braum, Cassiopeia, Diana, Elise, Fiddlesticks, Fizz, Galio, Gragas, Heimerdinger, Jayce, Karma, Karthus, Kassadin, Kayle, LeBlanc, Leona, Lulu, Lux, Malzahar, Morgana, Nidalee, Orianna, Ryze, Swain, Syndra, Taric, Thresh, Twisted Fate, Veigar, Vel'koz, Viktor, Xerath, Ziggs, Zilean, Zyra
I would literally be completely happy with Teemo if he was added to this list. Not only would I be happy with Teemo but he would be less toxic to play against in some circumstances since he would be able to be built full utility and low AP. People practically never buy morellonomicon on him anymore because mana regen items scale off base mana regen. His mana regen is 2nd lowest of mages and average for marksman. Despite the fact that he has a low cooldown high mana cost ultimate which is the majority of his kits importance.
After having 150% mana regen on frost queen claim, I realized how much I missed building Teemo in ways that took advantage of mana regeneration and high CDR. Once the mana regen was nerfed, I was left realizing that Teemo was essentially ignored in terms of mana regen buffs because he was in such a strong position damage-wise back in patch 4.20. It was only then that I realized how big of a problem it was.