I still think they should air the 80's version of Astro Boy. Really, it's one of the ONLY Manga Entertainment titles that has never aired on Ani-Monday, and the upcoming movie would provide the best possible chance to air it.
One of the more unlikely, but still likely, things that I could see airing is Evangelion. Sure, it already had many an Adult Swim run, but it's still a classic that could fit well with all the other mech shows they've been airing lately.
In terms of newer stuff, I'd love to see Soul Eater or FMA:B. Heck, maybe even a Code Geass run when Adult Swim loses the rights.