The castle really looks nothing like Daein Keep to be honest. Also, the emblems on the flags don't match. It resembles a black dragon, which was the last boss of FE1 and FE3, the games Marth started. So I would say that Marth deffinatly returns.
Ike also has high priority, but I don't think it's wise to introduce a new character via a stage. Yoshi also got a stage, but he hasn't appeared yet, same goes to Marth I guess. Marth is also very populair and a retro character so I doubt he'll leave anytime soon.
The 2 characters on the picture are Sothe (also from FE9) and Micaiah. FE10 takes place 3 years later after FE9, so it makes sence Sothe is older. But, Micaiah is the Lord of the two characters, but only for the first part in the game... Ike also returns as a Lord, and he's a rather new character which got very populair in a rather short time in both Japan and in the West, so I'm pretty sure we'll see him in Brawl to.