Remove Spellthief's Edge
Firstly: I didn't spend much time thinking through all the problems that this would cause, so feel free to discuss them.
This is kind of a radical idea, but I think players should be rewarded for actually being team players and bringing a support champion bot. Not for simply bringing more damage because they didn't get assigned Mid.
Especially considering that Spellthief's functionality had to absolutely destroyed just so those champions don't get too far ahead, maybe it shouldn't be an option to do so at all.
Then champions like Brand, Zyra, and Lux can be entirely balanced around going mid lane instead, and ADC's won't have to worry that they won't actually get a support just because someone didn't feel like picking a support.
Then coin could be adjusted accordingly so that mages don't abuse it but enchanters who take spellthief's don't get unfairly nerfed just because a different class is abusing their item.