For the villain storyline I would like to combine Bragollach preparations (non-explicit, for the viewers), Thuringwethil's (last or second-to-last? We haven't decided when she dies next season) big spying operation, plus going into Huan's life choices and fate that come to the forefront next season.
I haven't written an in-depth account of this plotline yet, but I have worked it out in more detail compared to the brief mentions it got in the sessions so far. Thuringwethil, in the guise of an elf, is sent (either by Sauron on his own or on Morgoth's orders) to the fortifications at the Pass of Aglon, wich is a strategic location: The only way get into Beleriand between the Pass of Sirion and Maglor's Gap. In the texts it is heavily contested before and during the Bragollach. Therefore, she is there to figure out the fortification's weaknesses and gather information about Celegorm's and Curufin's troublesome cavalry force and their plans. If we want to make it more explicitly about Huan at this point we could make him a target of the spying mission as well.
During the course of this mission, Thuringwethil will look around the fortress and stay close to Celegorm and Curufin when they talk about strategic matters. I'm still a bit fuzzy on this next "doom reveal" part, but this is my current idea: Celegorm has a dream about a hunting trip in Valinor with his best friends Aredhel and Huan. Aredhel rides ahead, takes a turn and disappears (maybe into a forest that looks like Nan Elmoth? Into a tunnel that looks like the entrance to Gondolin? Depends on how we handle her storyline in detail). As Celegorm and Huan go looking for her, Huan is suddenly and viciously attacked by a vaguely-shaped giant wolf and yelps in pain (next to a waterfall? Maybe we limit the foreshadowing here.). The dream ends with a short vision of Mandos, looking at the Noldor as he spoke the Prophecy of the North, before Celegorm can react and he wakes up agitated. Huan is there to comfort him, but when Celegorm mentions what happened in the dream, Huan looks sad and turns away to a window looking north, confirming he has insight into that there's some truth to their destined separation. Curufin and Thuringwethil overhear the noise - Curufin goes to visit his brother while Thuringwethil listens silently (Maybe they go somewhere for the conversation and alert her that way? It probably shouldn't look like a totally lucky coincidence that she can hear them at this moment.)
Once again, what they say concerning Aredhel depends on how we handle the knowledge of her fate and the timing of the plotlines, but regarding Huan Celegorm expresses his worries that Huan will be killed by a mighty wolf for going with him and falling under the Doom of the Noldor. This knowledge will be brought to Thuringwethil to Sauron and Morgoth, who figure out Huan's doom and each prepare for it. Sauron wants Draugluin to seek out Huan the next time they battle while Morgoth, without Sauron's (and probably the viewer's, apart from a possible vague hint) knowledge starts raising Carcharoth, a puppy of Draugluin's line, personally to make him into the anti-Huan guardian of Angband.
This sets up the positions the characters will be in during Season 6 and gives us good building blocks for the Bragollach, where the Pass of Aglon falls (at least temporarily) and Celegorm and Curufin flee to arrive at Nargothrond in Season 6. Draugluin won't battle Huan (possbily because Celegorm is afraid and keeps Huan away against Huan's will? It might set up a first rift in their relationship) and their fateful encounter is delayed until they meet at Tol-in-Gaurhoth where it looks like Huan will give his life for Luthien like Finrod gave his for Beren...Except he totally kicks Draugluin and Sauron's behinds and is awesome! Also I kind of want Luthien to kill Thuringwethil at that point, but that's more of a spicy idea for discussions in the far future