Just remember that if we don't give the Feanorians anyone to talk to, they will have to have reasons to visit each other in order for them to be on screen.
This is more true for Caranthir and Amras than for the other four, who live in pairs. But it's a good point about all of them. There'll be councilors, thanes, captains, majordomos, etc. In actual conversation it's awkward to talk to people who have no names, repeatedly for multiple seasons. And throwing names about in dialogue doesn't necessarily mean the named characters need more than very brief characterization.
The Fëanorean hangers-on will likely remain shadowy, for a couple of reasons. One, we have enough brothers to keep track of that any extra people will just be cluttered. But more than that...the Fëanoreans get less and less sympathetic as we go (and they aren't exactly angels to start with), so why anyone stays loyal to them becomes a bit of a question.
This is certainly true for most of them, and the sons of Feanor themselves provide the stories of formerly-mostly-decent people who end up as warmongers/mass-murders. Celebrimbor is the main face of the folks who go to Nargothrond and then desert from Celegorm and Curufin.
Still, there are others in Nargothrond, Feanorians who die in battle before the Second Kinslaying, and Feanorians who help sack Doriath but then desert during the Third Kinslaying. We might want to name those who kill the sons of Dior, and/or those who abduct the sons of Earendil (if they aren't the same guys). There are also survivors who stick around in the Second Age (in Eregion?). We may need to flesh out some of them, or at least include them as doing things, in various scenes.
They're only staying loyal to each other over the Oath, it seems. We might want one non-Fëanorean 'Host of Fëanor' character to be an outsider point of view at some point, but if we don't need that, we probably won't elevate anyone above an extra.
I think the brothers are loyal to each other, except Celebrimbor and sort of Amras. Curufin tries to undermine/disobey Maedhros sometimes but isn't deserting him during the Fifth Battle. I'm sure they all love each other as family normally do, even Amras (one can love and hate one's sibling at the same time).
Why do their followers stay loyal to them during the Second and Third Kinglayings? Evidently most of these folks have as many morals as their princes (or less, since oaths of loyalty aren't magically-unbreakable). I don't have much sympathy for people who willingly serve warmongers and help them kill or enslave people. Some Feanorians probably take their oaths of loyalty more seriously than innocent lives, and it's likely illegal to desert. In a royalist society, people don't necessarily give consent to their rulers, they're expected to obey without question. The Noldorin royal family isn't tenuously on top of independent feudal lords who can easily challenge them, like in Europe. They're on top of vassal-lords who aren't their rivals, so those who aren't loyal have only one choice: desert. After the Nirnaeth, desertion becomes increasingly dangerous, since there are fewer and fewer places to go. Elves and Men do survive in little groups of outlaws here and there, but it's a desperate existence, and the risk of being taken alive to Angband is terrifying. That isn't a good excuse for killing innocent people, of course (nor a ringing endorsement of royalism in general). But there were Feanorians who remained loyal until the Third Kinslaying, and then either deserted or stood aside, so they apparently had or grew some conscience.
Edit: We'll also want to show Elros and Elrond's lives among the Feanorians, and I personally want to expand the later chapters of the Silmarillion beyond the bare-bones treatment they got from Tolkien. There might be
no other Feanorians who ever interact positively with the twins, except for Maedhros and Maglor. But will we ever want the twins to interact negatively with any Feanorians? If so we may want names and faces.
