I doubt they'll make a direct sequel to Radiant Dawn. Path of Radiance ended with many questions left unanswered (the biggest was the identity of the Black Knight) and the scene with Sephiran at the end of the game could be classed as a cliffhanger ending. Radiant Dawn pretty much answered everything, and left nothing major in the story unrevealed by the end of the game. Also, since the final boss was the Goddess Ashera, a direct sequel would require an even more powerful final boss, of which there is none.
The biggest reason of all is Ike's ending note, which explains that he left Telius shortly after peace was restored and was never seen again. A Tellius Fire Emblem title wouldn't work without Ike getting involved somewhere along the line, since he was the central main character in Path of Radaiance and one of two central main characters in Radiant Dawn.
Additionally, the special ending scene between Lehran and Ashunera was set 1200 years after the events of Radiant Dawn, by which point all characters, with the exception of Lehran and Ashunera, are most probably dead. There may be a slight chance of King Kurthnaga in his final century, but still. Lehran also said that there was peace for the 1200 years, but war is beginning to brew again.
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