Frankly, if we're down to 1, it should be Lucina as an Echo or just keep Robin. But I don't see Lucina ever leaving, and Chrom is as safe as Roy is. Robin has little reason to leave either, but is not as immediately easy as keeping Lucina.
Roy is the only one to even slightly worry about, and I don't see him in a bad position like before. I mean, if we were talking about a roster reboot size-wise, sure, but then again, Echoes might not be safe anyway in that regard(they're as safe as their parent is currently).