He's on the cover too. But if you think Orcus needs an opponent right now, you can always use his other main adversary, Graz'zt, who appears in the Manual of the Planes.
I though Graz'zt was a bit contrived and insipid when I first read about him (when I got S4:Tsojcanth in 1e), but I think the story line and atmosphere around him have definitely improved (I just re-read the Plane book today).
At some stage I was thinking of a souped up 4e conversion and mass expansion of I1wellers of the Forbidden City (loved the atmosphere). And was a bit taken aback by the 4e Yuan Ti being devoted to some one-dimensional snake deity, which was why I was musing on bringing back demo as their patron, with a nice flock of barlgura etc.
Now I would of course be very happy to hear that WOTC were planning to 4e-ise DotFC one day soon.....
cheers
Cas