I would think Pelleas could've been a much better choice for Radiant Dawn than Sigrun was. Nolan would've been a better choice than Edward as well, imo. FE8 did need Joshua instead of Marissa I agree, and Moulder could've been cut for Tana. FE12's selection is just silly, and FE2 is pretty bad. For FE12 they could've used Nagi maybe? She might not of been that important to that game, but it's hinted she might be a form of Naga who is a over aching character through the series.
I would've liked Xane somewhere too, though he doesn't have new art. Ellerean and Michalis would have been cool to have in FE12 too, Michalis is at least somewhat relevant, and Ellerean has the whole rival thing with Merric who is in the FE11 pack so that'd be cool.