I've seen quite a few concepts with lava, but it never really appealed to me as a Nightmare version of Celestia since it didn't really fit the rules of the corrupted alicorn we've already seen (Luna's - but still, with most of them being before Rarity and that 'canon' Nightmare Star design it wasn't a lot to go off of aside slitted eyes and flowy main), but I will admit I've seen some pretty good designs along that line - Lopoddity's comes to mind.