There is an ongoing survey on bandai's main website where fans vote for the Mobile Suit they most want to be given the Master Grade treatment. As reported by ngeekhiong, there has been a recent update in the Dengeki Hobby magazine. The top 5 are now as follows:
1) Gundam 00 Raiser
2) Gundam Double X
3) V2 Gundam
4) Turn X
5) The O
Voting is apparently still going on until April 30th, and you can vote here: http://bandai-hobbyproshop.net/gunpla30th/regist.php. The winner gets a chance to get MG treatment! Awesome.
Personally, I have a lot Gundam I wish to see get the Master Grade treatment, but I wanna post that list separately. But yeah, I could care less about the 00 Raiser, but the Double X and Turn X are definitely interesting choices for MG's. The Wing Gundam just got MG treatment so that dream came true. I'm about to make my own list, this should be fun.