Heres my 2 cp.
Location. There are a number of options for where you can put the RHoD in eberron. It really depends how you want to run it. The main places are
• Inbetween Breland & Darguun. This usually sees Dennovar or Brindol turned into New Cyre. This one requires the most amount of work as far as I can see.
• Endworld Mountains. MarkB (and I followed his lead), has placed the Elsir Vale in the Endworld Mountains inbetween the Q’Barran city of Whitefalls and the Blade Desert (Valenar/Talenta).
• Byshek Mountains: This has it inbetween Droaam and the Eldeen. Check Flame Drakes PbP at the Real Adventures threads on the WotC forum for further info on placement.
Essentially the further away from the centre of Khorvaire & the 5 nations you place it the less you have to worry about things like the lightning rail, message station systems, etc etc.
Me I’ve placed it in the Endworld mounitains, simply switched the churches to Sovereign Host and Silver Flame where appropriate, made the wild elves a disparate Tairenal culture (similar to the Valenar, but less war-like and with owls instead of horses).
The Dwarves of the Hammer Halls are Droranath in my campaign (one of the PCs is a Droranath).
As for the Aspect of Tiamat, you could make her a raja, or as I’ve done (again MarkB’s idea) she’s a deranged experiment from the age of monsters that went wrong (Demons vs Dragons war).
I’ll let him explain it if he drops by…