Haakon had put forward a fantastic idea about using Valinor to introduce the Feanturi, with them having been there this whole time, building their dwelling places. They have a clearer idea of their place in the 'Music' than any of the other Valar, and would be making the appropriate preparations. The other Valar could even be unaware of what they have been doing this whole time, which would allow us to show the viewers without weird exposition from the frame.
Melkor's reaction to the trees: He could perhaps see the mingling of light while both trees are active from atop Utumno and be stewing every time that light reaches across the sea. He might even feel like it is being withheld from him personally. This is a very natural reaction. Think about how you feel when your neighbor is having a party or barbecue, and you have received no invitation. Adding Melkor's resentment on top of that and you already have the seeds of his plot to take the light of the trees. He's just not sure how yet.
Fitting Namo, Irmo, and Nienna into the plot: When the Valar arrive in Valinor, if the Feanturi are already there, they can be the ones who begin to suggest a source of living light that they saw in the music. Nienna is quite easy to fit in, as she is instrumental to the creation of the trees, along with Yavanna and Varda.
Not super jazzed on that bolded point, Nicholas, it seems a little bananas that Yavanna would need the trees prophesied to her like that. Doesn't work for me at all. I dig Melkor's response to the trees a lot, though; he should absolutely feel personally attacked here. He had the lamps thrown down to teach everyone a lesson and show them they were useless without him, and now they've gone and made the light anew in his absence AND it doesn't really reach him all the way up there in the frozen North?
Unacceptable.
If you want to get Namo in on maybe doing some prophecy, I could certainly see him making a suggestion for where they ought to build Valmar, maybe. The idea I had for Namo, Irmo, and Nienna doing things went back to Vaire: the weaving of the first tapestry, which, per Prof. Olsen's spit-balling, would be sort of the medium through which Mandos reads the future. I'm trying to work the timing out in my head, but I guess we could basically have the Valar decide "okay, we're leaving this site of destruction at the base of the northern lamp to withdraw to the Western Contient", but once they arrive be at a bit of a loss for where to start. Vaire sits down and begins to weave and as she weaves, Mandos pronounces his "reading"as it were. Is that crap television? (I'll be honest, I'm not super proud of it.)
It seems somewhat of a no-brainer that doomsaying and rest would both be important to the Valar at this point, but how else could we work them in right now? Is Lorien doing any work on the rest of Arda at large? Isn't it kind of hibernating with the Valar withdrawing to what will become Valinor? I guess he really can't be doing too much or it wouldn't be much of a withdraw, would it....
Maybe we're getting ahead of ourselves. What exactly do we want to accomplish with Irmo, Namo, and Nienna? Healing grief would seem to be the general response to that question, considering who we're dealing with, but how mopey do we want the Valar to seem? They abscond to Valinor and immediately get to lamenting what's just been lost? What does proper mourning for this situation look like? Am I even on the right track with "Healing Grief"?