I keep stating to my friends that I think GW could easily represent different races within the four god specific factions:
Khorne - Duardin, a tough heavily armoured unit with low mobility or an artillery piece would work well with khorne.
Slaanesh - Aelves, I personally want Malerion to fall to temptation, but I am cool with plain ol' Slaanesh worshipers too.
Tzeentch - Beasts, Tzaangor already fill this role
Nurgle - Skaven, once again this role is filled already by Clan Pestilens, but they could be fleshed out within Maggotkin a little better.
This would need some work from lore writers and would need some more fleshing out ruleswise, but it would make the followers of each god feel more unique and provide more variety in terms of differentiating playstyle. Although I would be sad if they tried to imply that the different races couldn't worship the god of their choosing.
Although as a Slaves to Darkness player, I really want a return of the Hellcannon manned by Chaos Duardin. And I really want to be able to use it in my undivided army. Considering the awesome bound demon concepts in 40k I think this would look stunning with modern sculpting and in plastic.