well it cant really be a passing EU villain of the week (or even an EU villain of the 19-book series for that matter) It needs to be a prominent member of the permanent EU cast. Of those, not many possiblities exist to my mind. I would have thought Anakin Solo a possibility before he died, but Ben Skywalker seems to be the most plausible person to be powerful and skillful enough. I guess Kyp could at a push or Jaina/Jacen but they dont seem to have the necessary presence in the story. Mara could but it would make no sense for her character to be sent back in that direction so the newborn Skywalker seems the best candidate, if as you say anyone does kill Luke.
i also give 2/1 odds that Ben Skywalker will not make it through life without having a hand severed in a lightsaber duel.
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