I don’t think the designs will be noticeably different. A lot of what the 2013 cars will look like will be shaped by the 2012 cars, which in turn will have been shaped by their 2011 predecessors. Right now, the only thing we know for certain is that the 2013 cars will have smaller wings (assuming the proposal is approved). Otherwise, it’s too soon to say.
Smaller wings aside what we do know about possible changes essentially relates to how much room under the cars the teams will be allowed to exploit. This involve shifting everything in the cars up an possibly shifting the radiators outwards or upwards. Don’t expect old style ground effects cars but, possibly a weird combo between current indy cars, old ground effects f1 cars, an some of the top shapes we’ve got currently.
Also, expect exhaust blown aero to be massive in 2013, reduced revs, turbo’s, under floor aero, just the conditions in which they where initially conceived.
Smaller wings can mean a lot. Will they be narrower (smaller “wingspan”)? This would mean going back to a pre-2009 design of the wings. Will they be shorter (lengthwise)? This would probably reduce their efficiency the most, but it would also make them look very different from what we have seen before. Would probably make them look more like 1975-1985 cars…