As an Assassin you want to kill priority targets. As a Support you want to make your allies awesome badasses.
Make a Support/Assassin that gives a teammate the ability to be an Assassin.
Something along the lines of:
P: anytime an enhanced ally kill an enemy champion, your E resets.
Q: Cast on Ally: Ally's next ability deals x extra damage
Cast on Enemy: deals damage in an interesting way, be creative.
W: marks target enemy under x% health (scales with rank). It receives x% more damage from all sources.
E: Cast on Ally: the champion can press any ability to use a ground targeted dash. (Can cast on self)
Cast on Enemy: move yourself towards an enemy unit, dealing damage to every enemy in your path.
Increase damage and distance with rank, reduces cooldown.
R: AoE camouflage for yourself and allies. Your next auto attack deals bonus damage. Rank increases duration and reduces CD.
This one would probably be the case for the so asked for AD support