What do I want to see in 6th?
honestly, the list is pretty short.
1) No major rules changes. I think it works pretty well as is.
2) More regular codex updates. Update all the codexes within 2 years, then dont update any of them for however long it takes to implement the next 2 year plan. I know, its never gonna happen for many reasons, but thats what I want.
3) A new chaos codex and a new eldar codex first outta the gates

(well, a guy can dream...)
4) a progression of the timeline. Some major event or storyline thingy taking place. A new black crusade. A new hive fleet (eek!), the crons are coming for Mars, etc etc.
I have no illusions about any of the above actually taking place, but you asked what I hoped to see, so I answered.
As for the SoB stuff, personally speaking I think SoBs are cool, theyre well overdue a new codex, and it would be spiffy to see the imperium represented by them in the new box set (cos its inevitably going to be imperium vs somebody) The alternative is BA or possibly DA, and that doesnt bear thinking about. SoB are far cooler than marines, because theyre portrayed as being a bit on the mad side, and that makes them more interesting than the boring old marines, who have this "shiny" image. I know, technically theyre not, but theyre marketed as "proud defenders of humanity" whereas at least the SoBs are sold on the basis of their "buuuuuuuurn, burny burny burn em all!" philosphy. There's also a lack of SoB players out there (due to neglect, more than anything else) so it could nudge a large number of gamers into collecting a new army.
As for chaos in the box set, theyd make an ideal protagonist for the SoB, and its just as easy to make CSM minis as it is to make SM minis.
so, thats my call - SoB and chaos in the box, and not too many rules changes.
*edit* could be SoB & 'crons. I could live with that, too.