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I suppose it's possible, if the game really took off, that we'd see more levels, additional cardsets, and so on.You could also adapt D&D classes to higher levels of play. The current origins are a lot like races, after 10th, characters could embark upon a martial or psionic class from D&D, for instance. It'd take a little work to adapt them, but it could be done. You could also expand the list of uberfeatures, and just give characters 5hps and another uberfeature, each time they level, more or less indefinitely. (You might want to stop giving them +1 to everything, if you want to keep challenging them with existing monsters, though...)
Rather than trying to adapt 4E base classes for GW characters 11-20, I would adapt the Prestige Classes found in sources like d20 Apocalypse and d20 Future. The Nuclear Nomad, Evolutionary, Lawbringer, Salvager, and Road Warrior all make great classes for levels 11+, have plenty of post-Apocalypse flavor, and I don't think they'd be any more work to do.