Regarding Lúthien, what I primarily have already on her is that we want her to have a role, but we don't want her to be window dressing
OR to spoil her later awesomeness.
So, she can't just be dancing in the woods. But she also can't be outside of Doriath winning battles.
The balance seems to be for her to have a quieter role that maybe not everyone perceives. I did like the suggestion (I think it was Eliza's) that she could 'calm things down' after the Kinslaying reveal. That's the kind of shocking news that gets everyone throwing their arms up in the air and reacting loudly. Like, yes, it's a betrayal, but it's also horrible and unthinkable and a lot of other things. People aren't just in mourning over the deaths of the Teleri...they're angry at the Noldor (rightfully so!) and angry people aren't known for making the wisest choices. What the Sindar do...does not strike me as an angry response. It seems to be very considered. Recovering from that is a process - it's not just something you make a decision about and then never have to deal with or feel again. So, if we wanted to show her skill at healing, we could show her healing her people in that sense.
Her influence on Galadriel could be very positive. Galadriel is Melian's pupil, and it's not like Lúthien is sitting in on these lessons. Nor is it likely that Galadriel and Lúthien would have much in common or be 'natural' friends. But if we're showing Galadriel go from almost completely shut down and horrified over what her people have done (and her own ambition's complacency in at least going along with part of it), we can show her time in Menegroth as a time of healing not just because of her time with Melian and with Celeborn, but also with Lúthien. Lúthien may encourage her to create again, to trust that impulse in herself to 'sing of leaves, of leaves of gold.'
Morgoth (the Shadow) cannot create, only mock. Lúthien is very powerful, so we have shown flowers blooming at her feet and the halls of Menegroth filling with light as she danced into them, and her song (along with Daeron and Melian's) banishing spiders. Whatever we do with her this season (and next) should be in those veins.
But...specifics.
- What does her relationship with Daeron look like? Does he try to woo her?
- Does she have any 'adventures' or travels this season? Does she go to the Mereth Aderthad with Beleg and Mablung? Does she visit Nan Elmoth in the Eöl flashbacks? Something else?
- Does she manage to thwart some plan of the villains? If Sauron has been working up to the Kinslaying reveal to spread division among the elves, is it her efforts that help them reconcile and move on as allies?
- What happens when she meets her 'cousins' (Galadriel, Angrod, Finrod)?
- Her use of magic?
As long as whatever role we give her builds on what we've already done (Lúthien as the champion of the arts and the heart of her father's realm), and lends itself to pointing in the direction of her adventures with Beren without giving any of that away, we'll be good.