boy, where do i start? i'm usually busy telling people the actual canon information about them that nobody knows because only like 20 people played The Mageseeker, lol.
(Examples: I am so very tired of people saying Sylas is twice Lux's age, he's twenty-seven and she's twenty. I'm also tired of people saying he groomed her when that was very much not grooming. We only see him do one actively manipulative thing to her (weaponize her guilt for being a Crownguard to pressure her into getting Durand's notes), and then he stole her magic. Yes it was a dick move that traumatized her. It was still not grooming. He also regrets hurting her, apologized for what he did, and has given her her space ever since.)
The following headcanons shift based on what AU they're in. I feel like a lot of these show up in my fic War of Hearts, because that's my vision of how things would happen in canon.
Lux confesses first. Because it's going to take a long, long time for her to warm up to him again, let alone trust him. In Mageseeker, Lux was in Sylas' thoughts often, and he was nigh-constantly thinking about how he missed her and wanted to apologize to her, but ultimately he decided that it was more important for him to give her space. (Sylas' deep respect for Lux is one of the big things that really got me shipping them, lol, and Lux's mutual respect for him is literally the rock-solid foundation for any good enemies to lovers tale.)
Lux fell first, Sylas fell harder.
I think it'd take a while for Lux to start using petnames, but would eventually settle into "honey" and "dear". Sylas uses pet names way more, lol. Obviously has "little light", but also likes "sweetheart". Maybe a "honey" and a "princess" in there occasionally. Like that one tumblr post that's like "princess (derogatory) to princess (pity) to princess (begrudging respect) to princess (affection)". In the Anima Squad AU I really like Lux as a battle dove like Seraphine, so Sylas uses "little dove" for her there.
Neither of them can cook much but Sylas is still a better cook lmfao. Lux lived in a household with professional cooks, and any cooking/baking she ever did was just for fun. She only ever worked off of recipes. Sylas didn't get a chance to learn cooking at all until he joined Leilani's rebels, and Leilani did teach him how to cook her soup. His education was more focused on the basics of making food taste good, so he's a lot more flexible than Lux is. ...He's also less picky.
Sylas struggles with emotional intimacy, but he still enjoys PDA without shame. Lux appreciates PDA because she avoided touching people for so long out of fear that they'd sense her magic, so once she has someone she feels safe with regarding her secret, she gets to indulge. (Not to mention that even if she did start losing control of her magic, Sylas could easily absorb it and not get hurt.) Lux can get embarrassed by excessive PDA but still enjoys it.
I don't often imagine my ships with kids (because kids shouldn't be a given in any relationship), but I think Lux would want kids. Not anytime soon, as long as Demacia needs her for other things, but maybe a few years down the road she starts really considering it. She reminds me of those girls I knew in high school who'd fantasize about their future kids and choose names for them, but she never dwelled on the fantasy for too terribly long because of the whole "keeping her magic a secret" thing making relationships difficult. Sylas is a lot more resistant to the idea of having kids at first. Yeah, he's got Rayn, but Rayn was already old enough to take care of herself, somewhat. Sylas doesn't like the idea of a child's entire life trajectory on his shoulders. His own parents kinda screwed him over for the sake of honor and duty, after all.
In Anima Squad, until Lux officially ends up in that universe, I imagine that she and Sylas have a solid friendship. Lux is constantly trying to get him to be less reckless in battle, but Sylas is too busy having competitions with Jinx and Vayne for who can kill more primordians.
I have a modern au wip fic where Lux is working part-time at the front desk of a library while she's in college, and she gets intrigued by the punk guy who comes in and checks out all of the books that keep ending up on banned lists. Sylas is in a punk band with Jinx and Rell and Lux is pursuing a nursing degree because her family pressured her into it.