When? 
Don't know about others, but I certainly can't wait. 
Hope it comes out before Realms of Power: The Arcane (or what it might be called).
I can't wait for it to come out too. My saga is centered on a Merinitae!
The magical realm book will probably be called... ROP: Magic or magical.
Arcane sound good.
-Bellysarius
Well, since we already got the old "faeries" supplement for inspiration, and know already at least a little about the fae, I'm more concerned about the "ROP: Magic" supplément.
Anyway, these books come out in summer, if I remember correctly. So, you should have it this summer, or next. Something like that.
David Chart or John Nephew are really the only two able to answer this question, but I believe David or John previously stated the likely next book or books after HoH: Societates. You might want to run a search of all their posts, if you're really super interested.