Humhumhm...adult reimob is more difficult for me, more of a mix of choppy visuals than anything concrete,  but I'll see what I can do. I'm not including possible kids for now!

I like to imagine Reigen got a ring so early Mob was 16 (hiding it so well it wouldn't tumble out if Mob's powers went off during sex) so when he finally proposes it doesn't fit Mob's finger.

Mob already basically lived at Reigen's tiny apartment before they chose to move together. Reigen's relieved Mob's old club friends carry a lot of the bigger furniure. Less happy all of Mob's other friends talk about him like a geriatic old man. Ritsu comes over every day for the first two weeks to make sure his brother's not negatively affected by living with Reigen.

Marriage probably comes pretty soon after～ There's no reason NOT to marry early, they know they'll only want to be with oneanother. Mob brings up the idea first and is the one that eventually asks if they should just do it. They have a small ceremony after but I can't decide if western or traditional. Mob is so cute in shiromuku...I love to see it...
A lot of RMR like Reigen taking on Mob's surname, but I like the idea that they'll go with Reigen. Imagine not calling Reigen Reigen. I couldn't do it.

Years of married life together and the first time Mob notices a gray hair is when Reigen's kissing up his side, and hearing that startles Reigen so much he actually gets out of bed to see where it is. Has another quick crisis about becoming older and fearing Mob won't want him anymore if he starts actually going gray.
(Mob thinks it's so hot he's a little dizzy. He's got more stamina than shishou now and he WILL ride that old man until they're so exhausted there's no energy to worry about unnecessary things)