I love you for this ask and I apologize because you're probably about to get way more than you bargained for.

I'm not sure SJM has created two characters who so strongly mirror one another. Rhys and Feyre were similar but they had noticeable differences to me in terms of their personality. Like Rhys really struggles with forgiveness and letting go of the past while Feyre is pissed at 3 pm, then over it at 3:04 pm (with Tamlin it was more like 3:55 but she's even moved forward since ACOFAS while Rhys is still holding a grudge as of SF). 😂. Feyre is also more open while Rhys still tends to hold his schemes close to the vest.

But Elain and Lucien. They are these two characters who have both suffered immense losses and violations to their bodies. Yet they still internalize their anger and sadness and don't lash out at others. I think people mistakenly believe they don't have much depth as a result but I actually think it's because of their reactions that prove how much they actually feel. All the times they aren't talking, yelling, or demanding to be seen is time they're spending turning everything over in their head. They're also observing the world around them, they're watching the interactions of others. And all that time they're spending simply paying attention adds to their knowledge of how relationships and society in general function.

They have also both ended up in places that they did not choose for themselves. Their responses to that are what I'd consider both a strength and a weakness. I love their optimism and attempt to make the best out of any situation they find themselves in but at the same time, they can be too complacent as a result. They haven't learned to fight for what they want and where they want to be and instead just allow life to happen to them.

Some other very similar core values they seem to share is a respect for tradition (I actually could provide text from the books to support all this but that would end up being extremely long so I’ll spare you).  They have both mentioned an awareness of traditions and values of a certain place and why it’s important to uphold them.  

They both share in the ability to apologize for having made mistakes and we actually see them make changes to their behaviors showing that they've learned from those mistakes. 

It’s a weird thing to note but neither Lucien or Elain have said “Fuck”.  Lucien has said “Shit” and “Hell” and Elain has said “Hell” but I find it interesting that they both share a similar manner of speaking and neither is what I’d call crass or vulgar.  Speaking of “Hell”, they actually insult people in a similar manner and make similar jokes.  After the Human Queens left, Elain said they could all “burn in Hell” and Lucien told Amarantha she could crawl back to the shit-hole she came from (we find this out after he tells Feyre about something that was sent (Amarantha) from the shit-holes of Hell.  And they both made a joke about Amren being cranky.

They are both extremely social, good at talking to people, enjoy parties, enjoy the outdoors, and neither is afraid to show love and affection to their loved ones.  Lucien told Feyre she was a better friend to him than he was to her (I don’t necessarily agree but Lucien felt it) and Elain acknowledges that she failed Feyre (it's note worthy that it happened before Elain even has a book) but realizes she’d always been the foundation that held them up.  And both Lucien and Elain have held Feyre in their arms to comfort her at different points in the series.

As far as their story, I think it will start in Spring. Lucien is permanently stationed there as of SF and while I’m not entirely sure how Elain will end up there, there seem to be definite hints that it’s coming.  I have this weird head-cannon that I can’t seem to shake that Elain is going to feel the drums of Fire Night and find herself pulled there because the magic is calling her to restore Spring (and she’ll possibly perform in the Rite with Lucien though I could see a few problems with that, one being that if the Magic chose someone for her (even if she were unaware of who that Male was), would she again feel violated as a result?  Or would she feel connected enough to the lands that she was comfortable with it.  If she is an owl shifter than her physically get there would be easy enough. I've also considered that the book will open with her staying in the townhouse to give herself some independence after some of what happened in SF.

Even if Calanmai doesn’t happen or doesn’t happen right away, I’m still certain she’ll be going to Spring where she and Lucien will begin interacting.  I think they will both be extremely instrumental in getting Spring back on it’s feet, helping the people believe in the leadership again.  Once that happens, the armies will be in a stronger position to join in the upcoming war efforts. And I do think the reason Elain will be capable of this is that she's been gifted Goddess of Healing like powers to restore a fallen land.

I also see them traveling to the continent to help free Vassa.  It’s possible Koschei is Valg and from TOG we know that fire can defeat him though I don’t think Lucien’s fire can stand up to the lake where he’s at.  There’s a line in SF where Briallyn states Eris’s fire wouldn’t have withstood it so I think that’s a hint of something that will be a problem for Lucien.  Either this would be an opportunity for Elain to step up and use what I think are her healing powers (again, something else we know that can defeat the Valg) or maybe along with Elain’s powers, Lucien will discover more of his Day powers.  I’m not 100% if they’ll actually defeat Koschei or just manage to free Vassa while Koschei finds a way to escape from the lake.  If SJM plans on bringing the Asteri to Prythian in the final book (and depending on what number book Elucien’s would be) than I could see her having them defeat Koschei in their story since the Asteri would then be the new big bad.    

Major conflicts would be both Lucien and Elain addressing their past loves and finally letting go of them so they can move forward.  Elain learning to let go of her stubbornness over something she believes wasn’t her choice and realizing that even with accepting her destiny, she still has a choice in it.  I don’t think Elain will act quite as hostile as Nesta did towards Cassian but I have no doubt we’re going to see a little fire from her when she first starts interacting with Lucien. She might touch upon how her sisters and even Az view her as someone in need of protecting when she no longer needs it.   I also think we’re going to witness the emotional bombshell that is Lucien discovering who his real father is as well as making some sort of peace with both Eris and Tamlin.  I’m not sure Lucien will ever have the same close relationship with Tamlin as he once did but I can see them finding peace with one another and then maintaining a casual acquaintance in the future.  I do see his relationship with Eris growing stronger though.   

Really, there are so many loose threads surrounding both Elain and Lucien.  She would take their story in so many directions.  (Will they stay in Spring for awhile or head to Day? Will Helion be High Lord for years to come or will SJM find a way to have Lucien take over? Could Lucien step up as High King? ). Personally I don’t think the Autumn Court will play a huge role in Lucien’s book only because I feel like it would make more sense to be a focal point if Gwynriel end up together (what with her unknown grandfather and all and Eris, looking to take over the throne, as an ally of the NC.  But still.  Lucien does have ties to Autumn, Spring, Day, the Human Lands, etc. and Elain has always wanted to travel.  They both have unknown powers, they’re both sort of the underdogs of the series, they are the only Mates of the series who instantly knew they were Mates and had the bond snap into place.  

I could be way off base with all of this but I just hope that the one thing I’m not wrong on is an Elucien endgame!

Also, I apologize because I was rambling with my thoughts and it might all sound a bit convoluted!