What Hero should I build next?

Anna, Nino, and A!Tiki all seem like good options.

For a unit who most likely will be required for quests, do Anna. Her and the other Askr fighters often have quests involving them.

For a universally versatile unit that you’d probably use for a long time, do Nino. You have top notch IVs. She is a staple in most Arena teams. You really can’t go wrong.

If you want to venture into a BST arena team, do Tiki. She’s a tank on dragon teams, and kills anything that isn’t blue (literally, just about anything she fights (including some builds of Lucina and Julia) that isn’t blue will end up dying if they initiate combat and you run a proper build).

Hope that helps.

Nino. She's got her best IVs

Nino and Anna I'd argue are the best out of these. That S!Camilla could work w/Raven and TA.

With Nino I'd run the standard Gronnblade+, Fury, Desperation
For Anna I'd run the same, so it's really a matter of preference

Both Nino and A!Tiki are solid choices that won’t require much investment to function well, and can function amazingly with heavy investment

Nino is probably the golden standard for blade tome users, high Attack and Speed makes it so she can kill almost anyone when buffed. She may not have access to class buffs, but her statline is good enough to where she doesn’t need it.

A!Tiki is a near opposite to Nino in terms of use. A tank that almost never fails. Making her +Def takes her Def to extreme levels and somewhat frees up her breaths refinement, since her Def is already high, it doesn’t need to be higher, making it so you can make her into an amazing mixed tank if you invest properly.

Out of the two, I would personally go with A!Tiki. I am heavily biased towards her, but she’s the one unit that has never let me down. Nino, while still an incredible unit, is just not a type of unit that fits my playstyle. If you want a player phase monster, Nino is the way to go, but if you need an enemy phase monster, A!Tiki should be your choice.

What are you looking to do? Clear more single player content, my move up in arena, shore up a damage type or color deficiency on your roster?

I think Nino is kind of a no brainer but your goals matter to the final answer.

I love my legion so much that it hurts. Heavy blade seal took him from a mid tier unit to an absolute man among boys. Recommend 10/10

I'd suggest promoting Anna as she's needed for quests and is interesting with her axe's built in escape route. I never did Nino as you'll find enough of her at 4* to merge her and she'll be beating her 5* stats if you can source a Gronnblade+ from somewhere, I got one of her with bad IVs in a banner but otherwise you'll need to promote one and merge that one into the +Spd 4*. She is amazing and certainly worth it if you won't be promoting more to get merged 5*s.