The Conan movies have actually been on my wishlist for a while also. Conan the Destroyer's dvd is just a port of the laserdisc, so it hasn't even been remastered yet. Maybe Universal will wait until Lionsgate puts out the new movie to release the old ones on blu-ray, but then again they might not want to wait that long.
Legend will surely get released on blu-ray with both versions at some point.
Sky, THX-1138 is on the table for a blu-ray release sometime in the next couple years. Someone from Warner said so in a chat that's transcribed at digitalbits:
http://www.thedigitalbits.com/articles/032409htfchat.html
I would love it if they included the original version (as far as I know it's up to them and not Lucas), but we'll just have to wait and see.
Something that's at the top of my list is Mallrats, hopefully with both versions. I got the tenth anniversary dvd back when it came out, but I didn't discover until months later that the dvd-18 had a scratch in it causing the movie to skip right around the time Brodie tells the story about his cousin on the airplane. I tried bringing it back to the local shop I'd gotten it from even though they had a two-week policy. They wouldn't let me exchange it. I was too lazy to ship it off to the studio for a replacement. The scratch was on the theatrical side and when Universal ports the hddvd over to blu-ray, it'll pretty much undoubtedly be the theatrical version they put out. So I could just hang on to my dvd for the extended cut on the other side and get the blu-ray to get a non-scratched copy of the theatrical.
For a slightly different reason, The Blues Brothers. My sister borrowed the 25th anniversary dvd from me but she keeps all her dvd's in a case and OF COURSE she jammed more than one dvd into several of the sleeves. Blues Brothers, a dvd-18 like mallrats, didn't get out unscathed.
I'm sure Universal will port Dune over to blu-ray at some point. Maybe they'll wait and see if Paramount actually greenlights the new Peter Berg project and then time the blu-ray to coincide with the theatrical release (although that's years away at the earliest, so maybe they'll just get it out to blu-ray sometime soon just like every other hddvd).
Lionsgate said way back in '06 that they were planning on bringing the Dune miniseries to blu-ray, but still no dice.
Oh, and The Abyss. We'll probably get this in the next year or so now that Cameron is finally finishing up Avatar. I never got the dvd after hearing it was non-anamorphic.