I really hope he is at the Rumble. Smackdown needs him. Badly. They need Umaga too, IMO
I don't get this grave concern about the current Smackdown roster.
There are four wrestlers at Smackdown who generate enormous reactions and are the lifeblood of the brand.
At one end of the spectrum is the savage heel (Edge) accompanied by The Big Show, and at the other end is the beloved face (Jeff Hardy) accompanied by ... well, to balance the books we'll just say Matt Hardy.
Middle of the spectrum are the two powerhouse wrestlers who don't need any distinct gimmick because they ride on their legendary status alone: The Undertaker and Triple H.
Scatter Mr. Kennedy, Umaga, MVP, Vladimir Kozlov, Shelton Benjamin, Hurricane Helms into the gaps between neutral and face/heel, and the scene at Smackdown is fine.
If Christian Cage returned to Smackdown, he would need to be face IMO to balance out the roster. And according to reports (which I might add are all speculation and don't seem to be spoiler-worthy) he is becoming a heel.
I would prefer to see him at Raw.