(EDITED and REPOSTED)
AAA thank you for asking very much :) the darker ones were definitely easier to come up with ideas for than the fluffy ones lol!!!

Fluff:
-Does it need to be said... Malleus STUFFS Silver with the most delicious treats. Especially when he was younger <3 macarons, pies, pastas, steaks, all of the HIGHEST quality. It drives him mad that the only thing Silver ever asks for is mushroom risotto 😭 he could give you more!!! He could give you better!!!!!!! Silver's still so cute about it though 🥰 he gives the softest little "thank you my lord" before digging in so fast and desperate, he puts Sebek to SHAME 🥰 Malleus loves to watch him eat <3
-They would adopt :) I don't like kid tropes that much personally but they would definitely be the first ones to adopt the strange little kid who ended up stranded in their kingdom. In my heart, it would be beastfolk or merfolk. Two lovers of different species bringing in a kid of yet another species 🥰 (oh my god. Wouldn't it be so cute if their kid was beastfolk of some sort of forest animal... Silver was practically raised by the forest and now he's raising a little kid with traits of those same forest animals 😭)
-They don't do a lot of lip kisses? They rub their noses against each other 🥰 they feel out each other's features, ghosting fingers over the dips and curves of their faces 🥰 they rest their foreheads together 🥰 oh my god, Silver gives Malleus butterfly kisses on his neck and Malleus purrs... 😭 that isn't to say they never do lip kisses, obviously!! They'll only see it as something romantic if it's a french kiss, though (don't take it too weirdly, lip kisses are just platonic in my family)
- (BONUS) Malleus and Silver both purr. It's a fae thing and Silver picked it up. End post <3

Dark:
-If the nation is at war, Malleus will send Silver to the front lines. He'll regret before he's even done it, and he'll regret it more afterward. He tried to avoid it by saying Silver had his role at Malleus' side, but Silver, ever eager to please and give everything he can, begged him to place him where he could actually be most useful... He didn't want his country to burn while he did nothing :( He would threaten suicide if Malleus refused him, honestly (not that he would follow through, but at that point, Malleus would still concede). Silver dying on the field would be the last straw for whoever they're at war with. They won't last against Malleus' wrath...
-Less of a headcanon and more of an au, but come on. Every single au where Silver is arranged to marry Malleus from birth <333 so delicious. Lilia is grooming him for the role really early on, way too early. I'm saying he would cast magic charms that widen and self-lubricate his little butt hole starting when he was 5... Well, he knew he would grow up fast!! The sooner he gets these things over with, the better, right? He's only 12-14 when Malleus actually elopes with him, too... It definitely adds to it if they know that Silver is a prince from another kingdom. It's a power play. What better way to show dominance over another kingdom than for the future king to have their beloved lost prince, barely entering puberty, whimpering and crying while Malleus stuffs him...
And despite the fact that he was groomed for it so early, they'd probably forget to tell Silver what exactly is going to happen too 😭 I generally believe that Silver was so interested in knighthood and training as a kid that he wouldn't think to ask questions about love or sex, so he goes in BLIND. One day he's playing with Sebek in the woods, the next he's told to dress in these fancy robes and woaaahh what do you mean he's moving into the castle? What do you MEAN he's marrying Malleus??? Ooh... The poor boy.
-Silver has a lot of scars from when Malleus first tried to handle him as a kid. Burn marks, claw marks, even spots where he got practically butchered by Malleus' horns. Baby boy is lucky for modern magical medicine 😔 Malleus doesn't even regret it. While Silver was a baby, he definitely cared a little for the kid, but he was very much "well, he'll die soon anyway..." about it. By the time he cared enough about Silver to worry about hurting him, he decided that his past scars were all very attractive. He left his mark on him!!! He's his!!! Most of them were formed before Silver can remember, so he can't properly resent Malleus for it. He just wonders why he shivers so much tapping the tip of Malleus' horns....