it would be cool to see a marksmen support
edit: please stop talking about ashe. shes not a support, thank you.
maybe their passive allows them to auto attack allied champions which heals them for a small amount or give them a shield and either of these will scale off of ad or ap (we dont relly have an ad support do we....)
their q could be an attack speed steroid that allows for either damage or more shields/heals to allied champions and passively increases heals/shields like the new 
maybe their w maybe a movspeed buff that can be cast on self or allies which also gives 100% on hit omnivamp for next 3 basic attacks
their e could be a form of crowd control be it a targeted slow which also slows attack speed by a considerable amount at high ranks
maybe the ult could be something like twitch's ult where the heals and shields would get super increased effectiveness and high range
could also have unique interactions with on-hit items like sheen or wits end or nashors
so like a sample full build could be

and synergy with all sorts of other items depending on how their scaling is
