I'm pretty much in line with everyone else's thinking on the main character re-dos. We've really gotten a lot of ultimate (or close to) versions over the past few years, but there are some that could stand some updates or at least some little tweaks.
Han Solo - The VOTC Han remains one of my favorite figures of the entire line, but at the same time you never know what they could improve on. I'd be happy if they just got the original one out there a little more often, I can't believe it hasn't been recarded in the line somewhere - and I hope they start to. Outside of that, I'd like to see Brown Coated Taun Taun rider, and new Carbonite Han Solos (pre and post). They could also redo the Yavin one, since the last one wasn't right at all really.
Luke Skywalker - To me, Luke has gotten a lot of love in the past few years. The Hoth one could use some tweaks to make it perfect, but the recent Bespin, Jedi, Snowspeeder Lukes were all pretty much perfect. I'm happy with the Resurgence Luke for Tatooine as well, and I'd like to see that one recarded like VOTC Han (same goes for their previously done Stormtrooper versions). My most wanted one is probably a new Dagobah Luke as well. The OTC one was cool (I still like it), but they could really go all out with that now. Other than that, I could see a new Endor one to go with the bike.
Princess Leia - The ANH re-do is my most wanted one. At the time I really liked the VOTC version (it is still alright), but I'd really like to see them do this one from the ground up, all-new. Another "iconic" version of the character that it is hard to believe we haven't seen on the pegs for so many years. I'd like the Bespin Escape version too (like Scott said, an easy kitbash), and the Ceremonial one could use an upgrade. Endor for the Speederbike as well.
Chewie - VOTC/EB is perfect, just put it out there more. Another one that should be out at some point each year (or at least every other year). Too bad the line can't run like the vintage days and just cycle some of these ultimate ones back out there again.
Lando - I'd like to see the Bespin one re-done. VOTC is alright, and seemed to be retail poison, so we'll see.
Obi-Wan - I wouldn't mind seeing the "slicked back" hair version from AOTC, with the movie (likely) next year. Otherwise, I'm pretty happy with Obi-Wans from both trilogies. Slight variations (like the "burnt" version from Mustafar) would be fine.
Anakin - Again, pretty alright with the Anakins. They make slight improvements to the Evolution versions, and that pretty much covers that character pretty well. The Med Capsule version is still out there, but maybe not very likely.
Padme - There are always Padmes to do (or re-do), and I'm game for any of them. That said, if the line ended tomorrow and this is what we'd got, I wouldn't be too upset either.
Palps - New ROTJ one with throne would be the ultimate here, for me.
Vader - Overall pretty happy I guess, we've had so many. The removable helmet feature is nice, but I sort of like having some Vaders where the helmet/mask doesn't fall off all the time too. Again, we've had several pretty nice ones, so we should be set.
R2-D2 - Pretty much hit the high with the Resurgence pack version, keep putting it out there. They'll make more though, and I'd probably pick them up. Always enjoy R2.
C-3PO - BAD version, or if they could get the vac metal stuff down, as others have mentioned.
Overall, looking at the main characters, there are some that could stand re-dos (or just new takes to see what they could do), and others that just need some slight tweaking to fix up. But, looking at the figures I have on display in our office here, most have been done pretty well. Then again, we thought some were perfect years ago only to be improved upon again, so you never know what they could do. Overall though, I'm pretty happy with most of them. Just a few from the OT would finish them out pretty nicely for me.