What to do about Daeron?
We do need to start figuring out his story, and where it's going.
I do not want him to be someone else proposing marriage to Lúthien this season. That is going to feel redundant with both Beren and Celegorm doing that, as well as 'why now?' when they've known each other for ages. But more importantly, I don't want Lúthien being in a position of figuring out how to reject his suit when Beren arrives.
Naturally, Daeron loves Lúthien. Everyone loves Lúthien. And they are best friends. But what form does his love take?
I would suggest that he be someone who is in love with Lúthien, but is resigned to the reality that she does not return his feelings. He has told himself that he's fine with being friends, and that he understands that marriage to him is just not a thing that Lúthien wants in her life. He's content to play music for her and watch her art grow. He values his role in her life, and in the court of Menegroth.
And so, in Episode 1, when we are getting the 'outsider view' of Lúthien, someone can tease Daeron about when he and Lúthien are getting married...to which he responds something along the lines of 'never' or 'not likely.' Perhaps at that time we can see that he is not opposed to the idea, but he knows it's not what Lúthien wants.
And then Beren shows up, and Lúthien falls in love in Episode 4.
Suddenly, Daeron is not so content to stand by and watch. He is hit out of nowhere by jealousy over what cannot be his, but what Lúthien apparently is capable of. Maybe he thought she was never going to fall in love with anyone? Or maybe he was okay with it as a concept...something that would happen 'eventually' or 'someday', but the reality is a little too close and personal and brings out some feelings that weren't present before. He can't fool himself about being 'over' his feelings for Lúthien...he's clearly not as resigned as he thought.
So what does he do? In this situation, I think he would warn Lúthien against Beren first, as a concerned friend...and only when she ignores his advice does he go to Thingol and reveal Beren's presence in Doriath. That conversation leads directly to the audience where Beren proposes and Thingol gives the quest. So while Lúthien may feel hurt by the apparent betrayal, all Daeron accomplished was hastening the thing he was trying to prevent.
During the Treehouse imprisonment episode, Daeron would try to make up with Lúthien. He never meant to hurt her, he was only looking out for her best interest, etc. And as part of that apology, he aids in her escape.
After she has left Doriath, which he knew she would do, what happens to him? Is he imprisoned by Thingol for going against the king's orders? Does he regret his choice and worry about Lúthien's safety out in the world? Or does pretty much nothing happen for him until Celegorm's proposal arrives? He could be angrier at that than he was over Beren, because he would know that Lúthien does not love this guy, so the sheer presumption and gall....at least DAERON respected her wishes! I would think this would be the moment he'd be spurred into action.
Tolkien sends him off hunting for Lúthien, ,never to be seen again. What do we wish to do with him?