Lucien is not going to have a HEA, at least not in the traditional sense
He has been cucked, emasculated, exiled, etc. Some things are legit sad, and for that, I feel bad for him.
But many of these are prime examples of why he won't be a MMC.
He talked back to Amarantha and she took his eye.
Beron killed his first love and possible mate
Rhys made him kneel and called him "Fox Boy"
He is no longer welcome in the Spring Court
Feyre saved him from Ianthe
Cassian and Azriel saved him and Feyre from his brothers
His mate isn't interested in him
He didn't fit in in the Night Court
He left to find Vassa and stayed in the mortal lands (where Nesta would have been sent to essentially be exiled), thus he's in the Band of Exiles
He lives in Elain's ex's estate
Mor said he can't be trusted
He doesn't appear to have great fitness levels despite being chiseled
His mate shrinks in his presence and does not seem to enjoy his gifts
His mate was aroused by and ready to kiss another male while he was in the house
Despite the power of flame, we have rarely seen him use it, which makes me question how powerful he actually is
He doesn't know that his dad is actually Helion
I'm sure there's far more that I've forgotten.
The point is, he is not being set up for a traditional HEA, and he's not MMC material. SJM doesn't make a habit of emasculating and unmooring the MMC. In a non-fantasy story, he would be the character we would be worried about. The one who would be found in a gutter somewhere, who would meet a tragic end, or who would need surveillance because people are worried he would snap. And I recognize this IS fantasy so he could have a huge turnaround. But consider when Rhys was under Amarantha's control, he still had power. He was sexy and won Feyre over. We did not see him be emasculated. He was cool and cunning. Nesta protected Cassian more than once, but he got injured in the call of duty as a warrior. We didn't see him be emasculated either.
So when we see how the MMCs are treated, Lucien is not being set up to be one. He will not be a High Lord. He will not rule the Day Court. I suspect he will stay in the Mortal Lands with Vassa and become mortal or she will become Fae or be revealed as Fae.
But either way...I do not believe he will be our next MMC.