Our finale will be very villain-centric.
Melkor and Ungoliant destroy the Trees without any attempted interference from the Valar. No one notices they are there until it is too late. And so, the Trees can't really be right outside the door of Valmar, or it's going to seem silly that no one was able to come check this out as soon as the Trees started dimming.
Of course, once they *do* notice, everyone rushes to Ezellohar, and there is much wailing and gnashing of teeth.
Meanwhile, Melkor and Ungoliant (covered in darkness) go to Formenos. Everyone in Formenos either flees or hides themselves. The *only* one to stand up to Melkor in the darkness is Finwë...and of course he's immediately killed. We should make the most of that confrontation, since it's all we'll have of the good guys challenging the bad guys. Tulkas beating the air uselessly as the shadows prove too much for him will be...less than satisfying.
Melkor plunders the vault at Formenos, taking the silmarils and destroying any stray palantiri lying around.
The request for Fëanor to break the silmarils to restore the Trees should have plenty of dramatic tension, but again, this is like the *one* thing that the good guys do, and while they are ignorant of the attack on Formenos, the audience probably isn't.
We do get a messenger arriving from Formenos with the news of Finwë's death. Who is it? We do have to have Fëanor flee out into the darkness alone after getting that news, but I understand the desire not to make that be our last glimpse of Fëanor this season.
What is Ulmo doing during all of this? We know what Tulkas, Nienna, Yavanna and Aulë are up to, and Manwë and Varda should be taking a step back (to reveal their thoughts next season). But...what about Ulmo? He's been Melkor's primary foe for some time, and the Darkening seems like something he should react to.
The initial idea, when the season was first plotted out, was to end Melkor's actions here with the Thieves' Quarrel: his hand being burned by the silmarils, Ungoliant turning on him, and the balrogs coming to his rescue (to bring back that storyline from the first half of the season). It is, of course, possible to reconsider that. We could save all of that for the first episode of Season 3, to juxtapose with what is happening in Tirion and establish Morgoth as the primary villain in Middle Earth rather than re-introducing him already in Angband. If we chose to do that, this season would end with Melkor and Ungoliant escaping into the darkness, and no one knowing where they're going or what they plan to do.
Season 2 has been an introduction to the elves. So, regardless of where we end Melkor's storyline, we should come back to the elves at the end, I think (see Marielle's suggestion above). But other than Finwë, they've all been remarkably passive throughout this episode. So.....it makes it very difficult to 'wrap up' any sort of story with the elves. I'm very clear on what Melkor is up to in this episode, and I know how at least some of the characters are reacting to his actions here. But...what story are we trying to tell with this finale?