Since people tell me I never say anything positive, I just thought I'd throw my hat into the ring on what I would like to see should the Evolutions assortment continue past the first two packs (and we all know it won't, but still). This is based on the assumption that each figure will be a brand new, VOTC-esque sculpt with no action features.
-Luke Skywalker: Stormtrooper Disguise (ANH), Bespin Duel (TESB), Endor (ROTJ)
-Palpatine: Sidious Hologram (TPM), Senate Duel (ROTS), Throne Room Duel (ROTJ)
-Threepio: No Coverings (TPM), Moisture Farm (AOTC), Full Coverings with Removable Limbs (TESB)
Other than that, I can't think of anyone who has actually "evolved" over the course of the films, appearance-wise, to justify a three pack based on the concept itself. You could so several with just different outfits (which granted, the Luke is), but at the same time, like Valin said, do we really need/want the thousandth version of Han/Luke/Obi-Wan/Leia/Chewbacca/R2?