I always thought it'd be cool if there was a champion able to take mana away and deal damage to another champ, focusing more on the mana take away, it would become a new kind of poke
Kassadin had mana steal on his blades once, and it didn't turn out well. Obnoxious to play against, people just don't like being out of mana. Maybe a bit less bot lane, since you have ADC's who don't rely on abilities too heavily. But the core gameplay pattern is terrible; they steal your mana and you're left playing a boring right clicker.
It would be an interesting concept. Especially towards people who think playing a support is nothing but constantly spamming your skills (like healing or shields) it would help in harassing adc with large wave clear skills. Probably be very useful early laning phase. Once it's mid to late game it probably wouldn't be too helpful, unless everyone is just spamming their skills in team fights!
This exists in Dota, it'll probably never exist in League. I can't find the quote, but Riot explained their stance on it a long time ago. They feel that it's too binary (an AP champ either has mana and can do things, or has no mana and can do nothing) and that mana burn doesn't feel satisfying enough for the person who is doing it. There's also the fact that it would suck to have an ability that did nothing at all against the ~20% of champions who don't use mana.
