A Marksman Support?
*Disclaimer: This is just a concept I find interesting with some thoughts on it, I haven't crunched any numbers or finalized anything, basically I am just throwing this out there for y'all to do what you will. Feel free to expand on it or do something completely different.
With Riot now finding many new marksmen being taken into the jungle and into top lane, I had the thought that maybe there could be a possibility of adding a new champion that was a marksman support. Right now the closest we have to this is Ashe, who isn't really designed for the role and only provides cc. I was thinking of a champion whose kit uses basic attacks to debuff enemies and buff/heal allies, somebody who scales off of attack speed and AD but for utilitarian purposes. Some abilities I have considered... -(Passive) Champions basic attacks deal reduced damage but apply a stack on enemy champions when they are struck. Upon reaching four stacks Champion gains one unit of healing ammo that can then be fired on an ally to heal them (flat amount+AD ratio) and grant them a boost to AD for a duration. -(Q) Champion throws out a skill shot. If it hits an enemy champion, it marks them. If the mark is popped by Champions basic attack, the target is briefly stunned and the nearest ally is healed for a small amount and granted a boost to movement speed -(W) Champion throws out a grenade (like Graves smokescreen). All allies in the area receive a shield and all enemies in the area are slowed. -(E) Champion dashes a short distance and gains a short burst to attack speed. The cooldown is reset if Champion pops four stacks of his/her passive on an enemy before the attack speed buff has timed out. -(R) Champion roots himself/herself at current location. For a duration, champion has unlimited access to healing ammo and a boost to attack speed and range. Champion can reactivate this ability to fire a missile that when colliding with an enemy stuns them and bursts into a field that reduces the armor and magic resist of all enemies in the area and boosts the armor and magic resist of all allies in the area
Once again I am not sure if this is even viable or whatever but it's just an idea do with it what you will :)