Support-as-ADC Champion
It would be cool to have an ADC-as-Support, but I think it would need to be an entirely new champion designed for it, not just Ashe/MF/Caitlyn as a "Support".
I would propose something like, NotAshe (600 range):
PASSIVE (stolen from Kalista) - FRIENDS WITH BENEFITS: While NotAshe and her FRIEND are near each other, their attacks gain 10 - 27 (based on level)% bonus attack speed. Additionally, their basic attacks apply BENEFITS to their targets. If both apply BENEFITS within 2 seconds of each other, both FRIENDS heal for 10-40hp, capped against non-champions, and cannot endure BENEFITS for 10 seconds.
ACTIVE - FRIENDSHIP BRACELET: NotAshe begins the game with Friendship Bracelet, a unique item. Upon activation, NotAshe hurls Friendship Bracelet at the target allied champion, beginning a short ritual during which both NotAshe and her target are unable to perform any actions and her target performs their dance animation. Upon finishing the ritual, the target becomes FRIENDS to NotAshe for the remainder of the game.
Q - Disengage! - Shoots an arrow (skillshot) that pushes enemies away and roots them for 2 seconds, 25-12 second cooldown
W - Engage! - NotAshe dashes forward and gains a temporary health shield, 20-12 second cooldown
E - Watch Out! (similar to Ashe's Hawkshot) - drops a 30 second ward with one HP (like a blue), anywhere on the map, 3 charges, global range
R - Not Today! NotAshe trades places with her FRIEND, and for 5 seconds gains stats equal to the highest of the pair (ex. if you have more current HP than your ADC, you keep your HP, if they have more AD/Crit/AS than you, you gain equal AD/Crit/AS to them).
CAPABILITIES: she's a lane bully support kind of like Zyra who mostly works by pushing the enemy ADC out of lane or bullying enemy tanks/enchanter supports. She can do really well in quick trades by using Engage! to dash in with her shield, do some damage, then Disengage! to reset - but that also leaves her vulnerable to ganks (I'm thinking she's very low MS). She does the whole Kalista Oathsworn passive / combo stuff most of the time - and she has a great ult for either sacrificing yourself if your ADC gets caught, or ruining an ADC duel by tagging your ADC out when both get low (and tagging in to steal the kill!)
PAIRINGS: She could do pretty cool stuff when paired with Kalista, because they would be both FRIENDS and OATHSWORN, and they could do bizarre ult-trading things together (huck your NotAshe support into the enemies, let her tank some with her Engage! shield, then Not Today! to swap your Kalista in to clean up the kills. Or you could just be horrific lane bullies with Caitlyn combined with FRIENDS/BENEFITS. Or you could even run with NotAshe/Ashe for massive vision control, great lane control, great 2v2'ing, but weaker teamfights. NotAshe is a top tier babysitter support, but doesn't contribute as much to teamfights as other babysitters like Thresh/Nami/Sona/Raka/Rakan - who have AOE abilities/peels/ults and are not restricted to only supporting their ADC.