Next 5*+10 for arena core?

Fae is great because she counters Brave Lyn and Reinhardt
and her B Skills and C Skills are very flexible since she doesnt really need them to begin with, so you can give her whatever skills you need her to have

I’ve seen a lot of good builds for Fae that can combat Brave Lyn and Reinhardt and her resistance is pretty good, especially if you go for a resistance boon that she can absorb significant damage from mages with her high resistance and high HP.

As for Eirika, I have personal experience with Her as a +10 in my team with her new weapon refinement, and played well she hits very hard. However, my team has a lot of hone and rally skills which are integral to getting the best out of her build (everyone runs dual rally skills and/or a hone skill). If you don’t run a lot of rally/fortify/hone skills, I’m not sure how she’d perform for you.

Soleil can hard counter a lot of common high end arena threats. But it's unlikely that you will have 11 of her any time soon.