...what jacks would the other factions get?
Picking the Juggernaut for Khador's extreme sculpt was kind of a surprise to me. Does anyone remember the big resin models they made? Sorscha, Deneghra, a Juggernaut, and a Slayer were all represented. I would have thought that
PP would have gone with a model that didn't already have a limited run display piece.
Anyway, back to my original question, here's my guesses for
WM faction extreme sculpts:
Cryx - Harrower. As noted, the Slayer had a resin display version already in addition to the Seether, Nightmare, and Deathjack as different variants on the original, with the Deathjack already being kind of an extreme equivalent. This leaves the Reaper, Leviathan, or Harrower, and I think the Harrower wins out because it's on a larger chassis and its big claw lends itself better to a dynamic pose.
Cygnar - Centurion. The Ironclad is the safe bet, as thus far the extreme sculpts have been from their respective faction battle box, but ask yourself this question: What would look more intimidating on the table, an extreme Centurion or an extreme Ironclad? Yeah, I thought so. The Cent is also arguably more popular.
Mercs - Mangler. The Rhulic jacks are right out because only a subset of a subset of
WM players even play with Rhulics, and who really wants an extreme sculpt of a labor jack anyway? The Mangler was previously restricted to Magnus only, but MkII makes it a generic merc jack. Therefore, a large portion of the player base with access to it
does not have one. Instant sales. By Rule of Cool, the Nomad is also a contender (Giant ass sword or giant ass ball and chain? Hmm...), but I decided to give it to the older jack (release-wise).
Protectorate - Avatar. That way Protectorate players will stop complaining about their character jack being undersized.

Okay, let's try this again.
Manticore -
PP gave everything else the Protectorate does to the Retribution, what's one more thing?

Oh man, I got a million of these, but I think I'll stop there.
Guardian - It will look good upscaled and can easily be posed dynamically. The increased size gives the sculptor the chance to go nuts on the churchy designs. I think this one is the best choice, though the Crusader might make it instead.
Retribution - Hydra. I don't know on this one, but I like the beefy fists on the Hydra more than the wrist blades on the Manticore. And for some reason, the contrast of a highly sophisticated myrmidon with a feral, two open fist pose seems more striking to me than the giant sword on the Phoenix, so I counted that one out.
Of course, this is if they were all released today. Since they seem to be going at about a 1 extreme sculpt per year pace, things are subject to change.