If they get special order rules, They should have:
1)A Reds army (Like Raven Guard - "Lost huge amounts of warriors, needed more. Instead of the whole bio-genetic thing, they went in for more Redemptionists. May field an additional compulsory troops choice of Reds)
2)A "Heavy" army - Extra flamer or melta in a squad (costs slightly more points)
3)"Faith" army - extra acts of faith, misses out on something (probably reds, who aren't considered pure of heart).
4)"Traitor" army - work closely with Adeptus Astartes and receive additional weaponry from them. May take one tank or walker from Codex. Do not benefit from "Light of the Emperor" act of faith (or something else suitable) due to the distrust many of the sisters feel towards this order.
5) Commando army - Sisters working with Preachers to keep control of worlds under attack from cultists. Lose out on armor and tanks, but are fearless and something or other. Just imagine Sisters reduced to leather armor and ancient bolters... might be a cool army list.
6)Seraphim army - my god, we'd never live this down. Its right up there with Callidus, Vindicare, Necron Monolith and C'tan armies with all the powers of a SMurf. Oh well... best increase the cost by about a point to make it fluff-acceptable, right?
Well, thats my suggestion for the six orders (I know its off topic). Have fun talking about it or something.