Over at Enworld they have a discusson of the new book coming out in May, and so far the following is known: There will be a new section on the Planes, the Archdevils get stats, there are new demons, devils, and yugoloths, they are doing something with the Eladrin in Arborea (cross fingers...) and we are getting new Cambions. The book will be 256 pages (!!!), comparable to Tomb of Annihilation, and will have 130 pages of new (to 5e) monsters... It will have PC versions of Githzerai and Githyanki.
Edit: Shemeska the Marauder and the deposed Arch-devil Moloch are in the book. Also sounds like they are trying to reconcile the 2e and 4e versions of Eladrin by defining the originals as higher Eladrin (again, fingers crossed...)