We are going to have an entire Season of the show to devote to the Nirnaeth, so certainly the events leading up to it and the crucial charge that makes the western forces attack before the eastern forces are in position will be something we can discuss...in Season 7. People here may be less adverse than you think to the idea of making Finduilas a warrior. We've already shown the following women elves take up arms: Irime, Earwen, Galadriel, and Edhellos. We've divided elven society into 'healer'/'fighter' roles, rather than 'male'/'female' roles, so that while more men are fighters and more women are healers, there's not a strict gender role divide. In any given company of elven warriors, there would be some women in the mix, though less than half. This is at least somewhat compatible with Laws and Customs of the Eldar, where it is stated that young unmarried women (ie, non-mothers) could be considered physically equal to men (among Elves). We've also created a military role of 'herald' which would traditionally be held by a non-fighter. It's a second in command that is meant to bring a unique perspective to the leadership, and to prevent the society from becoming too strictly militant. 'Healer' roles in society include scholars and farmers; 'fighter' roles include hunters, not just soldiers. The only roles that will be strictly gender-exclusive are the 'bread-giver' is always female, and the High King is always male.
Regardless, if we want Finduilas on the front lines at the Nirnaeth, we can put her there...but I am not sure that we want/need her there. I would want to see what advantage that would give to our story, before eliminating the Gwindor-Gelmir dynamic. I realize that we do need to decide who is getting captured from the Fens of Serech in the Dagor Bragollach by the end of this season, so that leads us to answering this question sooner rather than later - if Finduilas is going to 'play' Gwindor's part, then Gwindor will be taking the place of Gelmir, and we'll have to show that. I do recognize that this suggestion solves some of the 'damsel in distress' problem from the original - now she is a hurt and damaged character, and the audience will think that Turin does not return her love because she is now unlovely, no longer the beautiful elf maiden named after the Pools of Ivrin that she once was. Is that a story we want to tell? Is that better than the love triangle with Gwindor? I am not sure it's an improvement, though it does give her more agency in her own fate. (And if Gwindor is being captured in the BoSF, then of course all of the Gwindor/Finduilas scenes would have to be in this season as well.)
Ange1e4e5 - as always, I must point out that whether or not you change something should not be the standard of whether or not an adaptation choice is a good one. The question must always be whether or not the change is improving the story you are trying to tell in the new medium. In this discussion, you've asked us to avoid 'fridging' Finduilas, which is certainly more-or-less what happens in the original story. Her capture/death is all about making Túrin feel guilty for failing to save her, and she basically doesn't exist outside of Túrin's story. Alcarlótë's suggestion of making Finduilas an escaped captive is meant to give her more story before Túrin appears. It's...an idea. It's okay if you don't particularly like that idea. But if you don't want to see her story go in that direction, then which alternative direction would you like to see it go in? Because if your answer is 'as written in the book'...then we are back to her death motivating Túrin again. Granted, I certainly acknowledge that there are other ways we could give her roles in the stories of Seasons 5, 6, and 7...if we choose to. But we would need to come up with that.
In the case of Avatar: The Last Airbender, they took a well-loved show with wonderful characters and then did...whatever that was...to it. I have no intention of ever watching that film, and based on what I'm hearing about the new live-action series, probably not much chance of giving that a chance, either, now that the creators have left the project for similar reasons. I have no problem (in theory) with further developing Fire Lord Ozai - he is a shadowy big bad villain whom is seldom seen on the show, so showing more of him or his motivations would (naturally) humanize him in ways. Since he *is* a human being, that's not a bad thing! I agree that he is not the type of person to show regret, though - that's not how you develop him further. Changing the character of others is much more problematic - Aang's peppy, child-like innocence is a very important part of his story (because he's only like 12!), so aging him up and making him into a moody teenager is...a different story dynamic. Sokka, as a non-bender, is handled very carefully in the show. Sure, he's comic relief, but he's never stupid, and they make sure that he's the person with sense in the group who is contributing in his own ways and growing in his viewpoint as he travels the world - early on, he doesn't think women can fight, and he doesn't think that anyone from the fire nation can be a good person. He has a very black-and-white view of the world which is connected to his limited experience of other people and places. As time (and their journey) goes on, he revises those views. Obviously, he's not the only character who grows. Zukko has a rather powerful redemption arc, and he's clearly the one who starts out as an angry young man with some rather serious father-issues...his zeal for finding the Avatar is all about his vain hope of regaining his father's respect. And of course....Uncle Iroh is perfect as is, and you can't mess with a character like that without enraging everyone who ever watched the show. So, yes, when something is very, very good as-is...you don't change it. You can add original content, but you must keep within the spirit of the original.
I should not have to point this out, but the last season of Game of Thrones was not an adaptation. The original work has not reached that part of the story, so that was all original content creation based on the earlier show and the existing books. I certainly agree that the creators of that show are not as good at coming up with original content as they seemed to think they are, but at the same time, I feel that the ending season of the show rises or falls on its own merits, and should not be viewed as an adaptation. You can't point to the original and say 'G.R.R. Martin did it this way!' I am familiar with three animes based on incomplete mangas that have had to take this adaptation model - adapt what is available, and then create original content to complete the story. The least successful was "Tsubasa Reservoir Chronicles" - that manga has an abrupt change in style, so that suddenly everything is terrible and much, *much* darker than the happy-go-lucky quest for the feathers seemed at first. It's all fun and games until someone loses an eye...(or in this case, accidentally time travels). The anime remains unfinished, because the intended audience was for a younger crowd that wasn't into bloody violence, and they just teased the main villain without ever revealing what the plan was. Hellsing was always going to be bloody and violent, but the creator of that manga took his time and only slowly published the story. So, the anime keeps the premise, but goes ahead and creates their own villain and tells their own story. It's...okay...but not great. Once the manga was completed, it was remade as Hellsing Ultimate - and that is a faithful adaptation. The story is better, though the ending is basically the same. (Here 'better' includes Nazi vampires attacking London by zeppelin in 1999, so, uh...'better' is a matter of perspective - it is very crude.) The most interesting is Fullmetal Alchemist. Very little of the original manga was written when they started that adaptation, so they had to make up a lot - including the main villain and the way homunculi are created and the actual consequence of human transmutation attempts. The anime is beautifully done, and the story told works very well for what they started with. But again, after the manga was complete, it was remade as Fullmetal Alchemist: Brotherhood, faithfully following the manga storyline. There is not as much creativity in this adaptation (almost everything in it is directly from the manga), but because it is based on the original creator's story, it has a bit more satisfying of an ending (to my mind). There is no question that the original anime is of superior style and creative output, though.
Our adaptation of The Silmarillion is also based on an incomplete story. We are going to have to deal with the fact that the story of Eärendil remained unwritten at some point. We are going to have to do a lot of original story work for the 2nd Age material. Granted, we know how the stories end, more or less. But how we get there is a question of interpretation, and the guiding creative views on this project (ie, Corey Olsen, Dave Kale, and Trish Lambert), have expressed a willingness to make changes to the story IF those changes further very specific storytelling goals. For the question at hand - is it important that Finduilas remain primarily the center of an unhappy love triangle and die as a damsel in distress as the main story we tell about her? Or would it be possible to develop her story in other directions, while still having her killed in the aftermath of the sack of Nargothrond in Túrin's story?