I hope they will continue with Sisters (just got my wife into 40K and that's the army she wants, awesome....except for the ridiculous expense and total lack of updates. That said I think they will re-release Sisters and agree with those above that they are an unretirable piece of the fluff. I could see GW doing a full plastic release and new codex to push it, maybe with Grey Knights / Inquisitors, maybe stand alone. Either would suit me. As long as I don't have to pay 100$ for 5-10 models. That crap drives me insane.