1) They could make an awesome (assembly required) dianoga with trash compactor setting. We have the stormtrooper Han and Luke figures, and Princess Leia (Imperial Prisoner). Some day we might get Chewbacca. Making the dianoga and trash compactor environment would actually be easier than Chewie, I'm sure.
2) The Wampa and TaunTaun are not creatures I'm going to elaborate on, and I like my Hasbro ones just fine, so I'm not in the market. But these are obvious and very do-able in plastic. If we go furry for the wampa, then we run into the "Chewbacca problem."
3) The Rancor is possible. With the market for the life-size droids out there, etc., a 1/6 scale rancor need not even be that big (as the lifesize C-3PO for example) to be accurate proportions with the 12" figures. It might not have to be poseable to cut its cost, but if it were not, I'd still consider buying it, as it should be in an awesome pre-pose as it is. Any articulation pushes this one over the top to a must-have!
4) On the rancor's note, SideShow could make the Sando Aqua Monster in scale with a fighter-craft assortment GunGan Bongo Sub for the 3 3/4" figures. I don't know if there is a market for this but it's something I'd buy.
5) The Eopiee might be possible - maybe even with a new re-issue of E3 Obi-Wan, or Qui-Gon of course.
6) The reek would be too cool = capable of carrying Anakin, Padme, and Obi-Wan all at the same time. It need not be too much different in size from the dewback. I see SideShow making this one.
7) The Nexu? Sure. This is a very simple one to do that they can package and price at the 1/6 scale. that would practically be almost the same size as the figures.
8) Boga for Obi-Wan to ride? Way cool. It would be larger than the dewback though - I think. Longer anyway, because of the extensive tail.
The reek, wampa, and tauntaun are all naturals. The rest could be fun to visualize in your imagination, but less probable.
Stuff like the orray will never happen.