My solution is similar to @JPjr, in that I would group vanilla dwarfs, whatever elves don't fit the new factions and humans. But humans would be made from new kits that actually fit the AoS aesthetic, while I will squat most of the old Empire range. I really think historic cultures don't have a place in this setting.
Having said that, I understand that would suck for many Empire players, and judging by GW new approach, they're more likely to be satisfied than me. So hopefully they won't harbor bad feelings towards me!