Well, sure there will be some new 4E stuff, but so long as it is stuff that fits relatively seamlessly, than it's all good. For example, I can imagine Dragonborn (perhaps with some modifications) being renamed as Pterrans in Dark Sun (or perhaps as offspring of the sorcerer kings) - that would be pretty seamless. Eladrin is a bit harder, but still doable. Tieflings, IMO, just don't fit. Sure, it may be possible to stretch and contort them somehow to make them fit, but it would be, IMO, no longer seamless. As to new classes, Warlords fit just fine - no modifications needed whatsoever. Warlocks are seamless, but may fit with some restrictions on who they can get their powers from - either the elemental princes, or perhaps they could fill the role of the Templar along with 'evil Paladins' (good Paladins don't fit)...
In terms of Cosmology, the Elemental Chaos fits just fine, other planes vary in how much they fit.
The Dragonborn could be the Dray, could they not? Conceivably they could reset the timeline to the original boxed set while retaining new areas introduced in the later products.....ie. The Jagged Cliffs region, Eldaritch, Ur Draxa, the Kreen Empire, etc....
Banshee