Skyrim's marriage system has very little appeal to me. looking up the benefits of marriage on UESP it's decent but nothing spectacular.
also as far as i know none of the vanilla spouses offer much if anything unique should you marry them. they may make for interesting characters but as spouses, one potential spouse is hardly any better than another.
while there are some interesting follower mods that probably make for good spouses there's 2 reasons i don't want to bother with them. first i already have 3 follower mods and adding more would just break the game's balance. having that many already kinda does so i had to take some steps to address that. second is that i don't know if their marriage system offers anything more than vanilla Skyrim.
so anyway, here's an idea i had. a new character gets added which can't become a permanent follower. he/she would be a traveling merchant looking to settle down. being a traveling merchant they would be proficient in Alchemy, Smithing and Speech. so not a fighter in any way.
now just because this new character is looking to settle down doesn't mean the player can marry them from the get go. rather the player would have to complete a questline where they learn about their future spouse's personal and business life. part of this questline would be about helping this character set up business deals to supply various businesses both throughout Skyrim as well as in other provinces.
this character can be married once their questline is completed and the player owns a home. as a spouse this character offers everything vanilla spouses do and more. here's a few ideas of what they could do.
they could have interesting interaction with adopted children and pets. if moved into one of the vanilla houses they could wander about and interact with neighbors. i've seen mods that splice already existing dialogue to create new conversations.
new world encounters could be added where the player encounters their spouse, they are a traveling merchant after all.
this spouse could offer radiant quests. here's a few ideas for such quests, gathering supplies, saving fellow merchants, delivering shipments, and maybe not all these radiant quests would be so honorable, disrupting the trade caravans of rival businesses... well that's all i could come up with for an evil quest.
finally since this spouse doesn't travel with the player they don't know about the player's adventure. during conversations at home the player could talk to them about their adventures. players could tell the truth about their adventures or embellish their tale if they want to.
the only other suggestion i've got is that the mod should have 20 separate files so that players could choose their spouse's gender and race. it would also be nice if these spouses looked good. at the very least this new spouse needs to be an interesting character. they don't have to be so well written that they blow my mind, my standards are not unreasonable, but they should be more interesting than vanilla NPCs.