I've also got a dwarf army I'll be porting in, so I've given this some thought as well. I originally thought I would just port over any old units to free guild/ironweld equivalents. However, two of my opponents field wood elves and old greenskins, and they might not wish to proxy.
I'll be proposing that some of the legacy ranges will simply be allowed to play as part of an army (they gain the CoS and Dispossessed or Wanderer keywords). That way, we can play with warscrolls of those units and have some actual variety in our forces. The old warscrolls/points don't seem particularly generous so I can't see any real balance issues arising.
In terms of realms then yes - according to CoS restrictions you'll be based in either life or fire. But you could still write the lore of your army as heralding from elsewhere. They just happened to pick up artefacts on a venture into Aqshy!