I think the absolute core will be retrocloned OD&D/AD&D akin to Microlite.
The next layer will be very much 3.5: powerful Vancian magic, feats for fighters, different class mechanics, spells will have specific conditions and details rather than a handy list of universal conditions and timers, 3.5 multi-classing, no roles, more skills. (I also predict no healing surges.) With some Pathfinder-like improvement of casters like first level at-will spells and cleric channeling.
I think things from the Fourth Edition like rituals, martial powers, treasure parcels and milestones will only be represented in sidebars and slim optional books. I also strongly suspect that in order to build a 4th style fighter you will have to sacrifice a lot of damage and feats to get a few cool but nerfed powers. But as long as the Wizards keep the Fourth Edition Character Builder operating, I will remain happy.