One other thing I have been thinking about are the multiple stories about various kinds of relationships that humans have with mythical creatures in myth.
That is, as an expansion on the concept of contracts and magic through relationships with a magical creature/spirit/god, you might somehow come into a potentially intimate relationship with some sort of spirit.
If you contract with a fairy or something, and flatter, sacrifice to, and generally stay on good relations with that spirit, they might eventually be interested in a relationship beyond just trade.
In the case of having a contract with some fire god or fire spirit king/queen, they might suddenly declare that one of their subjects has been spying on your progress, and grown infatuated with you, or after a particularly notable performance for the spirit, you may suddenly find yourself being "gifted" with the spirit king's daughter's hand in arranged marriage.
This could be anything from a wondrous modern fairy tale romance to a horrific ancient fairy tale nightmare depending on the exact nature of the spouse.
A spirit king of a realm too weird might give a straight female adventurer a princess to marry, and wonder what the problem is? Are you going to insult the king by denying him this wonderful gift... and when are those grandkids coming along? (Oh, and did we mention the king doesn't deal with women? It's just really hard to tell what gender you are in your species. If you say you're actually a woman, it's off with your head...)
The proposition might be either absurd - your new spouse is a wise-cracking fire-breathing parrot cartoonish sidekick more than a romantic option - or something to be feared - one of the 7-headed acid-drooling forgotten beast types of spirits - or perhaps some sort of bizarre combination. Maybe you're getting forced to marry a magma kraken that turns into a beautiful spouse by the light of the full moon... or the blood sacrifice of children.
Likewise, how well the potential spouse takes it can be a little up for grabs, as well. They may hate the idea, and happily conspire with you to get out of the marriage. (And if we want to get downright Shakespearian romantic comedy, during the collusion, may grow to have feelings for you, after all.) They may just be a sadist who despises you and causes you pain every step along the way. They could genuinely have longed for you since spying you from beyond the veil since the time you were born, and hope that you'd be able to see past the fact they breathe ammonia and have six arms and fins for legs.
Trying to get out of a fae marriage could be an extremely interesting type of adventure, although one of the more esoteric to code for, as it would require having a much better conversation system to enable real diplomatic shenanigans.
On the other hand, if you aren't the sort to kick your lover out of bed just because she has gills and webbed feet, it opens up the prospect of using the "children of adventurers" mechanics planned to start a game as a descendant of deep ones, and innately casting magic.
In the case of magic spouses, it can also be an adventure just in every day life, as many magic spouses had contract-like clauses into their weddings, as well. For example, there are old myths of Celtic lore, where a king married a magical bride on the condition he never see her bathing. Though married for a while, curiosity eventually overcame him (as dramatic narrative dictates it must,) and he spied her in the bath, only to find that when submersed in water, his queen was actually a squid-like monster. She cursed him and fled.
Likewise, from Japanese myth, there are kitsune (spirit foxes) that shape-shift into humans and marry men and bear children that have magic powers - but fear dogs, as dogs can smell them out, and once revealed, they flee the village, and only return by night to watch over her family from afar.
Other Japanese myths (they're more into the "marry a magic creature" thing than most... go figure) involve yuki-onna (snow spirits) that nearly kill a man in a blizzard, only to reappear as a human, but warn not to see her at a certain time. When failing to keep his word, different versions of the story have the yuki-onna kill her husband, or else merely abandon him and her children because she couldn't bear to kill her own children, and breaks the contract off entirely, herself.
(It should be noted, these are usually predicated on the lack of understanding on the human's part that they actually are marrying a monster.)
There are also tales from around the world of various celestial marriages taking place where humans force a marriage between themselves and a known magical (often heavenly) creature by stealing the means they have to return to the heavens. (Often a robe of feathers that transforms them into swans or the like.) When stolen, the human (almost always a male) forces the celestial to be their spouse (almost always female), and the relationships often end tragically - often with a child of the relationship siding with their mother, and finding their mother's robe to give back to her. At this point, the celestial mother murders her human husband, flies back to the heavens, and abandons her half-breed children.
Ceres, Celestial Legend is an elaborately long version of such a story and its aftermath.
A variation of this takes place in D&D, as well, with nereids having their life tied to a shawl - if stolen they can basically become the thief's slave.