On the actual fluff itself, I'm always a big fan of role reversal although I find the results a little difficult to swallow (Druchii chilling out in Athel Loren what?), but that's probably because I haven't read the fluff. It's also worth remembering that everything is still up in the air: the End Times story arc hasn't finished yet and we're still to see how all the pieces are going to land. Just because ET:Khaine finished with the Elves united (ish) doesn't mean that's the end of it.
The only problem there is that GW is going to make themselves look very stupid if they change things again too much in another ET book. Different elf players sprinkling their armies with others elves and then GW pulls a bait and switch? That won't end well.
While the future of different special characters seems to be uncertain (well, those with older models (potentially by implication, finecast) seem to be the most at risk...), I'm not sure they'll do too much to the armies until new books come along.