A lot of people on here, I suspect, will say go for Magni. It may feel boring, but he’s really devastating on attack in raids and war. There’s a certain comfort in knowing you’re going to be able to knock out one enemy hero.
I have a similar-ish problem, in that having now completed PoV, my 6th scope in hand, I have to choose between my heroes on 3-70 (Vela, Alice, Isarnia, Lepus, Richard) with already maxed (Ariel, Magni, Frida, and Athena).
Personally, I think Ariel is spectacular and by far my most useful blue hero. If you have any chance of getting her, hold your mats. Magni may not be new, but he’s a destructive force. I can see why Fenrir would be tempting, but I’d recommend Magni.
I don’t think Isarnia is the choice, because while I ansolutely love her, you have Athena.
Richard and Vela are the best defensive options and Vela can be quite handy against titans with her crit buff and attack reduction to increase survivability.
I would pick Richard IF you have the costume.
But overall I would go for Alice. She is a strong backline on defense, has great burst for PvP and solid tile damage and great utility for titan.