Nathan P. Mahney said:
It seems to me as though the cycle will begin again. 3e started with a glut of Race and Class books, and 4e will do the same. I expect a few 3rd party Monster books as well.
This.
In the beginning people will be content with PoL or their homebrew, but they want to make the character concept that's in their head, and the PHB1 doesn't quite get them there. Third-party pubs (and later WotC with PHB2, etc.) will then fill in the holes in the conceptual character space with 4E equivalents of Barbarians, Sword-Sages and Duskblades. Once all the bases are covered though they'll move on to adventures and paths, as happened with 3E. It'll happen more quickly than with 3E though, since the path has already been blazed.
The earliest of these books will focus on new Talents or Feats and Paths/Destinies. Other than earlier-edition recreations I don't expect brand new core classes right of the gate. They'll come later. New Rituals and spells will of course be very popular, but of questionable quality in the first iteration.
In parallel I also expect the first round or two of books to have a lot of "implied setting guides" to stuff left a big vague in the Core Rulebooks, such as a Volo's Guide to the Feywild or, more specifically, Intrigues of the Faerie Court. They'll flesh out the various planes and settings for people who want to focus on one or a few parts of the PoL implied setting. For full campaign settings, only settings that are being "converted" to 4E, such as Kalamar or Midnight, will be released early in the process, and their "fit" with the 4E rules will be questionable. Only several years down the line will people have a firm enough grasp on what's missing from 4E/PoL to release a really good setting/ alt-ruleset such as Midnight or Iron Heroes were for 3E.
The wildcard in this product mix is expansion on the various "mini-games" within D&D. If the social encounter rules provide a good framework, but leave something to be desired, we may see a more Advanced Intrigue & Guile rules from someone. Likewise if the Leader classes present some intriguing mass combat possibilities, we may see some good mass combat (and siege) rules that capitalize on that pretty quickly.