Hi, so I'm going to get into why I feel Yuumi should get a rework, centered around her "W" ability but first a little bit of background on myself.
I've been playing League since late Season 1 and have mained every role and finally settled as a support Bard main, currently in D2.
Bot lane is traditionally a 2v2 lane where there's a chance to kill the enemy Support or ADC and since the addition of Yuumi you have become limited to only killing the ADC. Due to Yuumi becoming untargetable this takes away from the spirit of the 2v2 and feels incredibly frustrating to play against. Yuumi has been designed to be completely centered around her W ability and when not attached to a Champion she is essentially useless where it's kind of sad to see. Yuumi's "Q" ability too is another frustrating mechanic to play against, a "skill shot" that can be steered into you provides very little counter play and just adds to the frustration of facing against a Yuumi.
I would like to see Yuumi's W reworked but this would also mean the design of her Q to be changed too and as well her base stats to be buffed.
"W" should work as a Dash to Ally Champion (no longer attaching to them) providing a shield to them, the shield part of the ability would have a cooldown similar to Rakan's but Yuumi is still free to dash to Ally Champions with a short 1 second cooldown providing her with a good amount of escape.
"Q" would need to be changed too If Yuumi can no longer steer it due to not being attached to an ally, For this rework I'm suggesting I think Q should be able to be steered slightly left or right, if the ability is mid flight and if Yuumi moves herself left or right it would effect the direction of the ability, I think at maximum she should be able to change the angle of the ability by 45 degrees.
"E" is an extremely strong heal as it is, and with this rework I'm suggesting that it be toned down slightly as Yuumi would be getting an additional shield with the change to her W.
"R" due to this being an ultimate ability I think it should provide the untargetable effect, Yuumi attaches herself to an Ally and becomes untargetable for the duration of her ultimate and Yuumi can still cast this if there is no champion to attach too.
Done! I know I'm not the only one who feels Yuumi is very frustrating looking at Yuumi's ban rate which is currently at 40%, and hopefully I've shown some potential changes that could be done, going by the history of champions I don't think any of them have been reworked so soon after release so I doubt anything this drastic would come into affect. I imagine Riot will nerf Yuumi's heal ratio's as a fix but I think the problem is centered around her untargetable mechanic.