Why don’t you just make TP a keystone that focuses on sustain in lane

Make it part of the defensive tree so running into defense prevents offensive oriented champions (mages, adcs) that need offensive keystones from abusing it. It even takes TP away from lane bullies and opens up summoner spell diversity. Has some draw backs too, as it might be an escape hatch for champions who are weak early game into mid/late game - then again, they wouldn’t really have a keystone for most of the game: that seems like a fair trade off.