For me, I'd go with this for a Luke Evolutions:
-ANH Farmboy Luke... While he has a lot of figures, he hasn't got a defnitive version of himself in that outfit. Considering the importance of Luke in his farmboy clothes I think that's a sorely missing item in the line. His extras could include a (good) removable Stormtrooper belt w/grappling hook, stormie blaster, blastshield helmet, saber (clips to his belt), and softgoods poncho w/floppy hat. Lot of that stuff they already have sculpted, save for a good stormie belt really.
-ESB (gets harder to choose here) Bespin Luke... Luke spends most time in his ground pilot uniform, so it's my choice for an iconic "growth" of Luke look. Include a removable belt/holster, and if Hasbro wanted to get creative, I think they could pull of a removable Bespin Jacket. It would cover Luke on Dagobah AND Luke on Bespin. While some may not like that, I think that they could do it right and it'd look pretty spiffy to me. Include a saber (hooks to belt), blaster pistol... Removable hand for Bespin, of course... If they wanted to get really really really creative, a "damaged" head would be cool for his battered and bruised look on Bespin.
-ROTJ Luke... Personally I'd like superarticulated Luke with a removable softgoods vest. Underneath that he'd be in his Death Star duel duds (if they could make the chest flap a "heat/cold activated decal" like the damage on Stormtroopers, that'd be simply amazing). Pack in, obviously, his saber and a softgoods vest w/Endor helmet and that'd be Luke for all seasons in ROTJ really.
I'm amazed, at this point in the line, that a truly great DS Duel Luke doesn't exist, same with Bespin Luke who has had some pretty half-hearted efforts in my opinion. For that matter, Hoth Luke's been short-changed a bit despite a number of versions of that figure made.