Well with Fire Emblem DS being a remake of FE3, which Marth was the star of, I'd say Marth's chances of staying are about 95 - 100% now. As I've said before Marth is the icon of the series in Japan, not to mention Sakurai knows Marth is popular here in America because of Melee.
With Marth popular in both territories and being the star of the newest Fire Emblem game that was just announced for the DS, there's absolutely no reason for Marth to be removed now.