I suspect they're done with releasing classes for the existing power types, at least for the next couple years.* That means the Player's Handbook 3, which has already been in the works for a while, will probably include eight new classes from the four unreleased power sources: elemental, ki, psionic, and shadow. If they follow with the pattern of the first two books, there will be four of one and two each for two of the others; for instance, two elemental, two ki, and four psionic, with the Player's Handbook 4 including two ki, two psionic, and four shadow. (The PH1 had two arcane, two divine, and four martial, while the PH2 had two arcane, two divine, and four primal.)
Next year should see the release of Player's Handbook 3, probably pretty early in the year, unless they decide to release it in time for the holidays. The last two were released nine months apart, and the last one in March, so December wouldn't be unreasonable, but they haven't announced much of anything for past October yet—just the Draconomicon 2: Metallic Dragons and The Plane Below, which as yet has no description. (I'm guessing it's a book on the Shadowfell, but who knows?) I suppose there's a good chance there'll be another five races, as well, much like in Player's Handbook 2. It'll be interesting to see what new races and classes there will be, since part of the fun of creating a new character is mixing and matching the options available, and having more options means potentially getting closer to the sorts of things you really want your character to do.
I've also found that having more options can help me imagine a totally new facet to the character I hadn't previously considered. Considering a tiefling bard and realizing that, with the Spiked Chain Mastery feat, I could use a spiked chain as a magical songblade made me realize a number of things about the character. She spent a good deal of time in the Shadowfell and learned to use the spiked chain from her shadar-kai companions. Her enculturation amongst those people helped her make many uncomfortable realizations about herself, her own people, and her place in the world, which was one she no longer wanted. Her time amongst her own people, the shadar-kai, and the people she's met since then, as well as the myriad roles she's played amongst these groups, is why she has so many different kinds of experience—mirrored by her multi-classing—and her distaste for those previous roles is what lead her to become a bard as a departure from her former life.
*This is ignoring setting books like the Forgotten Realms Player's Guide, of course, which contained the swordmage as a new arcane class. The Eberron Player's Guide should similarly include another arcane class, the artificer. This is just talking about the core material.