The one Tahm change that needs to be made
There are a lot of complaints about Tahm on these boards from people who don't want to build mr, qss's, work on positioning, cc Tahm to stop him from shielding, etc. However there is one genuine issue with him that needs to be fixed.
Tahm's devour is currently coded so that it doesn't go on cooldown vs invalid targets. That is great in the jungle, especially with devour's hokey targeting. The issue that arises is when the target is an enemy champion instead of a jungle monster. If Tahm attempts to devour an enemy champion and is denied, the full cooldown should hit. Right now, noct, Sivir, morg, etc are all punished when they try to intelligently use their spell shield vs a Tahm because he blows through the shield and then eats them anyway a half-second later. This legitimately removes valuable counter-play.