Let’s think about this: in what way would Sauron and Morgoth, et. al., find out about Melian? Unless they can directly sense the presence of each other
without specifically looking for another Ainu, the only way I can think of is that Morgoth (having heard it in Valinor) knows that Melian is... somewhere in Middle-earth. Nobody in Valinor knew where, though. She didn’t emerge from Nan Elmoth until after the island had gone for the last time. But “Melian is somewhere in Middle-earth.” is no more informative than “Oromë likes to wander Middle-earth hunting my monsters.” What would lead them to specifically suspect, and look for, a Maia right here in Beleriand?
I think her presence will come as a surprise when she makes the Girdle. Speaking of which, will it be possible to show the Girdle in terms of shadow and darkness, not light? That is, instead of showing light pushing away shadow-spiders, can we somehow show wholesome, good shadow repelling foul evil shadows? That would be my preference.
Re: Dragons: dripping poisonous saliva, poisoned slime trails that kill plant life, poisonous blood, and
maybe poisonous fumes I can get behind. Glaurung does produce poisonous steam by immersing his boiling hot (and slimy) body in a river, which wouldn't produce the same steam if a non-fiery dragon did it. But the water would still be poisoned and defiled. What I want to avoid is the D&D depiction of Dragons as exhaling ice-bolts, explosions, and 30-100 foot long high-pressure streams of acid and contact-poison. It's really neat, but not an Arda thing IMO.
Side question: Do the sons of Feanor each have a "host" of elves who follow them specifically and live with them? Do they divide that way before Feanor's death? If so, what happens to Amras' people? Would he actually live alone?
Yes, every prince has their own group of people and followers. So they each have civilians, soldiers, captains, healers, artisans, and (presumably) friends who aren’t their family.
In principle like Brian’s ideas about Amros, but I want to be very careful that his story doesn’t repeat the character arcs of Maedhros and Maglor. Those two are in all versions (except the Lost Tales) shown as the brothers most reluctant to continue Kinslaying, and as-written the only two who actually repent, and who try hardest to get out of the Oath and
not fulfill it. This is apparently the direct cause of their surviving the longest. It’s part of what makes them more likeable (or less hateable) than their brothers. I’m not willing to take that story away from them, or have Amros just do exactly the same. I think at the very least there should be a clear distinction between Amros regretting the Oath without regretting the Kinslayings, vs. Maedhros and Maglor regretting and then repenting the Kinslayings but only more slowly regretting and opposing the Oath itself. All three do spend 26 years refusing to fulfil it, including 11 years of withstanding some kind of (apparently magical) torment, so they were all clearly actually quite dedicated to trying to prevent another Kinslaying and/or breaking the Oath. I just cannot imagine them doing that if any of them was OK with business as usual, or felt no regret/remorse. But Amros’ reasons should be unique, as is his response in the texts. Spite and maybe hate against the Oath and his father, and determination to spoil/oppose/undo his father's legacy, would be rather distinct. He can also be remorseful as well, but probably less than Maedhros and Maglor.
Another suggestion (in this thread??) that was rather disturbing (and akin to suicide) was to try to break the Oath
so as to join his twin Amrod in whatever fate befell him -- even if that's the Void. That's very dark and I would not want that self-destructive attitude to be present for most of the story, but it would work at/prior to the Third Kinslaying and make that story even more disturbing and tragic.
I’m OK with the idea that he ultimately concludes that fulfilling the Oath is the only destiny left to him – and we can show through the somewhat different behavior of Maedhros and Maglor at the Third Kinslaying that Amros was tragically mistaken about this.
I do especially like the suggestions that his interactions with his brothers, and with Men and Dwarves, ultimately helps add to the tragedy of betrayals the Noldor experience. It won’t work with Celegorm and Curufin (they crossed Dorthonion to Tol Sirion, and fled down the Sirion a few years later) but it can work in other circumstances.