Yuumie's kit should be more defense oriented. (rework suggestion)
Yuumi was designed as a support, but pros want to use her as a hitchhiking damage dealer, or movespeed steroid. This means the design failed it's purpose. Buffing damage just makes her a ksing support or 2nd adc, and buffing heals isn't very unique. Why not make Yuumi a sort of anti-cc, anti-gap-close, anti-burst support?
passive - "yummies" collect cat food to empower abilities (collectibles like Bard/Zoe); Yuumi's auto-attacks also deal bonus damage to monsters/minions. (compensation for no wave clear abilities)
q - "yummy" cleanse effect, consumes 1ammo, Yuumi and partner heal and gain +5 permanent hp; if breaking a cc, also cause a small aoe knockback. (cannot use with no ammo) w - "you and me" attach/detach; partner gains 30% of Yuumi's armor/resist and Yuumi takes 30% of the dmg; (similar to knight's vow but applies to new target faster) e - "boop" Yuumi flies like a projectile bouncing off an enemy and back doing a moderate knockback; empowered - consume 1ammo, deals bonus damage and slows enemy.
r - "final chapter" same ult, but partner gets 50% kill gold if Yuumi gets the kill (similar to Pyke's "your cut"); empowered - consume all ammo and gives partner a shield based on how many used. (AP ratio means Yuumi chooses between building AP or tank)