I propose a crack-ship and the couple's Carrot Cake and Rarity . Story would treat an affair, an affair of Mr. Cake tired of the monotony of living in monogamy in Equestria where polygamy is the norm and monogamy is seen as an eccentricity. The existence of a monogamous couple is due to tight control by the mare on the stallion which is the case of the Cake family.
Mr. Cake always wanted a big family as did his grandfather and father, but knows he cannot do it with a single mare and also the relationship is losing the fire of passion, the little romantic details are increasingly scarce, privacy is almost nil because Ms. Cake believe that intimacy is only for procreation (secretly she knows she is sterile and that embarrasses her a lot). All this claustrophobic situation is the limit to which Mr. Cake despite the great fear of his wife looking for himself a romantic affair with a young mare.
And Mr. Cake found a mare which reminds him why he fell in love with Ms. Cake in his youth: Rarity. A young, beautiful, hard-working and successful mare. He does not know how but tried to approach her, perhaps through her friend Pinkie Pie ...
That’s what I have so far. I hope the idea catches the attention of an author to make it a great saga. Nestra revered Celestia knows that Mr. Cake is a character who deserves a great story and I'm surprised he does not have such importance for the fandom .

My interest extends only as far as posting this humorous image upon reading the words "Rarity" and "Carrot".