I thought the 4 cyl 1600(ish) turbo engine format was pretty nailed down already. It's not exactly a return to ground effect either, as blown diffusers, movable floors & flexi wings are doing that anyway. It's probably just a shift in the balance from wing aero to underbody aero, which means more reliable downforce in the wake of another car.
With that in mind one interesting thing is whether they'll manage to get a reduction in wing size past the newly formed sponsors group, or perhaps the proposed tweaks to sidepods will make up for that one?
Also with allegedly around 650BHP is there a chance that GP2 could be quicker than F1 – AIUI one of the reasons the FIA don't want to take away things like exotic materials, paddle gearboxes etc. is that they're also in many of the lower formulae so it could be claimed that F1 is no longer the pinnacle of motorsport (technology).