I was thinking about this today too.
Some things to consider:
-Super Mario Bros. Deluxe was never released on cart in Japan; it was only available through the Lawson convenience store download system, which is now over. So the All-Stars versions of 1 and 2 could be an option. But remember that the SMA games have all been rather expansive since Mario Advance - Mario 3 was given a complete overhaul. As fun as they are, Mario Advance 5 featuring SMB 1 and 2 would not be an impressive followup unless there was some dramatic change to the games.
-With the spate of SNES rereleases, and the success of Mother 1&2 and the Mario Advance remakes, I'd be shocked if Mario RPG didn't hit the GBA within a year or so. I doubt it would be tagged with the Mario Advance title though.
-Yoshi's Story sounds like it could happen too. They've probably got a working GBA version sitting around already and are just wondering how to position it as a commercial success.
-Super Mario Land and SML2 would require top-to-bottom upgrades, and let's not forget that they are very limited games. This would defeat the purpose of the Mario Advance series if they had to be entirely reprogrammed and redrawn. A brand-new game, as well, would defeat the purpose.
I'd look to the SNES back catalog if you really want to know what's going to happen GBA-wise. There's two more DKC games. There's two Fire Emblem games. There's Metal Slader Glory Director's Cut. There's Marvelous, an RPG directed by Eiji Aonuma. Kirby Super Star. Star Fox. Hell, there's even Super Punch-Out! if you want to get crazy. I'd put money down and say that at least half of these games will end up on the GBA. Sadly, Nintendo's kind of running out of Mario titles. And I say that without a speck of sarcasm; I was completely addicted to Advance 4 these last two days.