In no particular order, I certainly wouldn't mind seeing any of these guys:
1. Angilas. Pretty much a no-brainer.
2. Rodan. See above, with the added benefit of public name recognition.
3. Varan. He deserves to be in a
good movie for more than 5 or 10 seconds.
4. Manda. I think he'd be a good choice if they wanted to segue into more fantastical elements; he has a pretty basic design, but they could easily work in his background as defender/god of Mu.
5. Biollante. Even if they do away with the soul angle, she's still an impressive monster, and would probably be amazing if done properly with CGI.
6. Mothra. Another no-brainer if they wanted to explore some fantasy elements.
7. Hedorah. Why not, I'm on the bandwagon now too.
8. Gorosaurus. Just a solid, down-to-earth monster design.
9. King Ghidorah, especially if he transforms from Monster X. KG's an obvious pick, but I feel the MX angle would provide an extra bit of intrigue and mystery for the non-fans in the audience while basically telling anyone who's in the know what to expect. Ideally, anyway, that's how it'd work. Of course, as soon as you tell people that MX is showing up, most will want to know where he came from in the series and/or what's so special about him. Mostly, though, I just put him on my list because I like Monster X.
10. Titanosaurus. Another good design that's not too bizarre for a first or second outing.