If Robin's Final Smash remained as Pair-Up, then I think Anna would be the best choice. Given how long she's been in the Fire Emblem series, I think she at least deserves that much. Furthermore, Awakening is the first Fire Emblem game in which Anna is actually playable. That said, what better way to celebrate her first playable appearance as Robin's partner?
This is exactly why I think Chrom would be an excellent Echo Fighter of Ike. Chrom's sword would be shorter and weaker than Ike's, but the lower knockback would allow for more juggling. Basically, he could be a combo-heavy version of Ike.
As for the awkwardness in attacks, they could easily tweak Chrom's animations so that they better suit his character, as they did with Daisy. For instance, Chrom's fighting stance doesn't have to be exactly the same as Ike's; he could stand and run similarly to how he does in Fire Emblem Warriors. Also, unlike Ike, Chrom could use only one hand for Eruption. As for the flames, he could use Naga's gold-green flames instead of Yune's blue flames. Lastly, his Quick Draw could look like the move he uses in the below video, at 0:10. Unlike Ike's Quick Draw, which knocks the enemy away, Chrom's Quick Draw could stun the enemy, allowing him to follow-up with a jab or tilt.