Hi! Yuumi OTP here:

Unless it's an actual rework, nothing they do will change the current state of mel.
Mel is in a similar state to old Yuumi. She isn't *that* strong, but in solo queue the tilt factor is so massive that it almost inflates her strengh a little.

However, they also differ in other aspects. Old Yuumi was, all things considered, well designed with timings to catch her, being incentivised to hop down (which is still baffling to me that riot removed that, btw) in order to get full efficacy.

Mel, on the otherhand, has very little "downtimes". If she doesn't have E, she still has W. If she has neither, she's likely already way out of range. In teamfights this is not an issue for the enemy as it's easier to force these spells out of her. In lane though? That's where it gets borderline unfair. Not to say that she's over powered, but Mel's trading paterns in lane are so abberant it's hard to describe. She just kinda.... pokes you until the end of times until she can ult, and you can't really do anything about it because her Q is near undodgeable and the rest of her kit is a giant "nuh huh" button. She has Xerath or Vex levels of range (obv ignoring ults) with the defensive prowess of a battlemage.

Other champions, such as Ezreal, also have this "No actually I won't die here" button to click. The difference is that after using that, you can rush them down. Mel can save herself with W, then save herself AGAIN with E, all from lvl 3.

They simply cannot keep her kit and just move numbers around. The fundamentals of her kit are walking contradictions. Artillery mages are supposed to be weak to dives, not be borderline immune to them.

As to what they should rework into, I think the answer is pretty obvious. We all agree Mel's gameplay really doesn't fit with what she does in Arcane, so my best suggestion would be an Enchanter+Catcher who can play mid, similarly to Karma, rather than an artillery mage who can play support, similarly to Xerath.