I don't like the idea of including actual game rares into starter decks as not everyone will have those cards. That would put them at a slight disadvantage since they'd have less cards to use.
Not having these rares puts them at disadvantage in almost all PvP events. They can always sell these cards, and looking at the starting decks it seems that selling/buying cards as your first step after entering WoE is almost as important as it is in the real game. For example a number of elements start with useless cards (deathstalker, dune scorpion without any attack buffing cards, quints could be used only on PUd, fractalled or mindgate'd creatures, etc.)
I'm also worried how

is going to fight mono without phase shield. Immortal/Phase Dragon rush without shields? Fractal sparks? Quite incomparable to eg. immolation+ash eaters+phoenixes for example.
Also, there is no thread to ask questions unrelated to any of the threads, so I'll ask here. Are there any special rules concerning Underworld players? Eg. in terms of economy, leadership, conquest, etc? Are they simply a 13th team?
And a related question, what happens when a whole team is eliminated (all their towns and cities captured)? Are they eliminated from the event? This would be quite un-MMORPGish.
I was thinking that maybe players from defeated elements would become 'underworld', like ronins seeking revenge for the destruction of their empire. So there would be no team underworld (although their skills could be accessible), until one team is eliminated, and then they would go around seeking revenge and stuff (right now it looks as if they were avenging some non-existent past). The only problem is how to balance things so that underworld doesn't grow too powerful in the end, and so that eliminating teams actually eliminates threats, not creates new ones. So far I have no good ideas how to do that.
What I like about this whole idea though is that it resembles some epic fantasy settings, where the old races are fighting an ever increasing horde of something (chaos/darkness/zombies etc. depending on settings, in this case rainbow

), and the end of an era is some kind of an epic battle where one of the forces is actually eliminated, and the whole world goes BOOM!, and then empires slowly rebuild and a new era starts.