Kael Colbane was the Svartalfar hero (and before that, an AV hero). I already asked who the new dark elf hero will be, but Kael hasn't given a clue about the new heroes that will replace either of the the new religions' heroes.
Ok, it seems I somehow read Overcouncil as Undercounsil, hence the confusion and the question earlier. It didn't make much sense to say that any neutral of evil civ can join the Undercouncil, but not be a member of the Undercouncil. Still, I think that a voting member/full member distinction would make sense for the Undercouncil, as would representation based on how many of your citizens follow the Council.
Still (if possible, which it probably isn't, without some major changes in the games code, some of which might not be accessible to modders anyway), I think it would be much more interesting if the an Undercouncil member with the Esus state religion could feign another religion/alignment and be a member of the Overcouncil as well. Perhaps in such a case his vote would actually be controlled by the Undercouncil, where his true allegiance lies. They would basically just be there to thwart the actions of their opponents councils, but calling a vote to throw out true good civs under the pretext that they are the Undercouncil's spies would be a nice twist. Any Overcouncil civ could be able to accuse any others of being traitors (kicking them off the council if the council as a whole agrees, and possibly having a small chance that their real alignment/religion would be discovered even before the vote, thus making their expulsion automatic), but only the real traitors would know if the accusations are right. You would get a huge diplomatic malus with a civ that you accuse (whether right of not), and if the Overcouncil disagrees with the accusation they will then vote on whether to expel you instead. If both the accuser and the accused are really traitors, then the accuser should also be kicked off of the Undercouncil (Unless you decided to make it so Undercouncil spies couldn't control their own vote at all, in which case perhaps the Undercouncil would hold a vote to decide what all their spies' votes should be. In this case it would probably be impossible for a traitor to betray another traitor). The ultimate aim of the Undercouncil would be to control the Overcouncil, thus insuring that good and evil civs alike are under their control. Failing this, they should want to make the Overcouncil collapse, so that once the good civs have been divided, they can easily be pick off, one at a time...