The ability to add multiple Pro Channels side by side (routed parallel).  By making the first effect in each channel a band pass filter you instantly create a multi-band PC.
 
Or even include the filtering by default as part of the PC header.  (switchable)

ddmf metaplugin

Yes,  I know about the various plugs that provide parallel processing hosts,  if the PC could do it we wouldn't need them.

it's a fairly grey, thin, possibly dashed, line between host and plugin these days...
what about running the fx bin in parallel? (rather than just post/pre) though tbh i'd rather the effort was spent making the host functionality super stable and leave the whistles & bells to the plugs imo

It would be cool to have the ability to switch to another tracks pro channel kind of like how ddmf's eq works. I should probably make that a post, but I'm too lazy...yawn.

I'd love to be able to process left and right of a stereo track with their own PC without having to split the track.

Id like to see Blue Cats MB7 Mixer and Patchwork bundled in X4. 
 
They are already in the Cakewalk store and would be a routing godsend. MB7 already works much better than any PC module could. 
 
 
http://www.store.cakewalk.com/b2cus/ProductInfo.aspx?productid=30-BCM71.00-30E
http://www.store.cakewalk...ductid=30-BCPW1.00-20E