I can't think of much that hasn't already been pitched that's truly essential, but I would like to see an SA Cpt. Antilles or Gen. Dodonna - R1 offers a nice excuse for those - and Colonel Yularen's cameo on Rebels gives him a boost, too, in my opinion. Both the TCW and OT versions are highly expensive figures, and the film version's never been single-carded, not to mention he's more striking than most Imperial officers, plus possibility of reusing parts. R1 also boosts Red Leader.
Bail Organa is a PT character who I think has earned a Vintage cardback. I'm also very fond of Count Dooku, and would re-buy a version I already have on a Vintage card, although I would hope for the Geonosis Arena version I don't have, or a new/kitbashed sculpt. I think a ROTS Tarkin would be a perfect opportunity to create something totally new, but also an excuse to alter the sculpt (skirt and color?) for the possibility of a new R1/ANH version down the line.
TFA/R1? Well, a lot of characters from those movies deserve the SA treatment, but nobody's sticking out to me as especially necessary but unlikely. I'm not super fond of the Gen. Hux character, but a removable hat and jacket would be pretty ******.
Also as a general note, a ton of 6" sculpts could use downsizing. I'm very jealous of the realistic Rebels sculpts - I would go nuts for that Hera, and while the SA Thrawn from 2008 is a favorite figure of mine, I think we could use a fresh crack at him, with the yellow epaulettes and a blue-er skin tone.
My intention in this post was to avoid what's been said, but scrolling up, it looks like I've been beaten to many of these as it is.
I suppose the point here is must-haves though, not stuff in general we'd like to see, and the true must-haves for me would be a better Hux, Dooku, Antilles, Dodonna, Yularen, and while I love the aliens from the background, none of the unused ones feel individually necessary to me, save maybe a new Hammerhead. I love the aliens, but nobody sticks out as better than the rest of the lot, so to speak.