What if duardin get the Kragnos treatment? Grombrindal could be a Kragnos-style model that can be souped into other duardin factions, and/or have his own army list that combines any duardin model, similar to Be'lakor's all-daemon list option. That way GW gets to satisfy all parties. There's also the fact that they're willing to give models different points depending on which army they're in (I believe it's Tzaangors that have different costs depending on if they're DoT or BoC, for example).
I honestly don't see why they'd fuse such radically different factions that they've done so much to separate in the lore. Mawtribes and Warclans made sense because the individual factions wanted the same thing and had the same goals, they just fought in slightly different ways. KO, Fyreslayers and Dispossessed have radically different worldviews and aesthetics. Just my two pence worth.