Estinien in story

[Theodric]

I think there's a real risk that much of what makes him so popular and memorable ends up being lost if he becomes a fully fledged Scion rather than a lone wolf who occasionally crosses paths with the protagonists. It's dangerous to keep a character around for the sake of keeping them around - especially if they undergo drastic changes to justify their return to the spotlight. It just risks alienating those who admire the character for what they already are.

[Yuriane]

It just risks alienating those who admire the character for what they already are.

I think the problem there is you're kind of ignoring his original story arc. His original story arc was 100% vengeance against Nidhogg and how that desire for vengeance has been corrupting him since childhood. And now with the eyes dealt with, likely permanently this time, there's no real way to keep him as he is and still have him make appearances.

True we do have the matter with that dark energy that enveloped him when he destroyed the cannon to question but beyond that without his character changing drastically for him to find a new meaning in life they would literally have to reduce him to essentially a deus ex machina character who cameos just cuz every two patches exclusively before screwing off again. Honestly we can have our opinions but I reckon that's a fate worse than having his character change and develop further to accommodate being a scion.

[Yuriane]

Although that being said I don't think Estinien would really need to change all that much. Give him some soul searching, maybe a desire to protect Ishgard or something, and bada bing bada boom you have all you need to justify him joining the Scions.
Even then joining might be the wrong word seeing as Alphinaud is liable to practically adopt him into the Scions anyway.

He can still be a lone wolf type character, he's just a lone wolf type character who occasionally tells Alphinaud that the warrior of light needs to smack a fool in Ishgard for one reason or another.