Even though she's loathe to admit it, it's rare for Regina to be able to pull one over on the imp that is Rumplestiltskin. She still counts having the bookworm/princess/caretaker in her dungeons as one of her most satisfying accomplishments.
But the curse that she's working on now... it has the potential to be so much more satisfying. It's the chance to screw with far more than the Dark One.
Though of course, she has every intention of royally messing him up as well. Of course she does.
He put together the bare bones of the Dark Curse and helped her figure out how to handle the particulars. Then he left her alone to create the town of Storybrooke, Maine and the lives of its residents as she saw fit. The idiot; he should've known better.
As a matter of fact, she's pretty sure that he does know better. He's probably added some little contingency to his cursed life behind her back just to make sure that she'll keep her end of their deal... But that's half the fun for her in their little game, doesn't he know that by now? Managing to outsmart him, to think outside of the little box that he likes to think that she lives in. She knows that he will forever think of her as the scared teenager he taught basic magic to, and she's happy to let him. It works to her advantage, really.
Take this Dark Curse for example. He can set up as many protections for himself as he likes... but, knowing him, he'll be concentrating on himself so much that he won't think of how she could hurt him through proxy. After all, he likes to think that he's untouchable, that no one can get to him or whatever shriveled remains of a heart he has by now.
But Regina knows better. She saw how he reacted when she told him that his precious caretaker was dead. He's not heartless, and he can still be hurt, one way or another. So, yes, even though her main purpose in casting the curse is to see Snow White's perfect happily ever after fall apart, she's definitely going to mess around with the imp's life as well. It's just too good an opportunity to pass up.
For a ridiculously long time, she considers marrying him off to some poor, unsuspecting girl, a simpering princess like the one that he's already fallen for, or an impossibly uncouth village waif who would drive him - as both Rumplestiltskin or Mr. Gold - out of his mind.
Snow White is, of course, the first candidate to come to Regina's mind. She's pretty, and dark-haired, and difficult, and too young for the man in question. That sounds like Belle, right? And if the Dark Curse were ever to - god forbid - ever break... She imagines Snow White and Rumplestiltskin waking up in bed one morning, realizing they've been made into man and wife and actually cracks up at the idea - until she imagines what Rumplestiltskin might just do to her entrails once he finds her. Besides, her real key to Snow White's unhappiness is that she has nobody to love. So, no, Snow White is not going to become the Dark One's wife, no matter how tempting the idea is.
Maybe the dog with the cape would work? Red Riding Hood? Hm... Then again, giving Mr. Gold someone as... appealing as Ruby Lucas is set to become just seems a little too... kind to him for the game that Regina's playing.
Mr. Gold, she eventually realizes, is just not meant to be a married man. He's meant to be as lonely as Mary Margaret Blanchard will be, and in some strange way she suspects that will bother the pawnbroker as much as it will the school teacher. But that doesn't mean that she can't twist the knife a little.
It's late in the curse-making process when she gets the perfect idea, and she waits until the last minute to add it in, just to make sure that the imp doesn't catch wind of it in time to stop her.
She knows the imp, knows that he has his own reasons for wanting this curse cast, but she's not dumb. This curse is no help to him unless it's broken... so she assumes that's what he wants to happen, and she prepares her own contingencies just in case he gets his way. She's always intended to bring Belle along as a part of the curse - to stow her away in the mental wing of the hospital with no memory just in case she needs a way to keep Rumplestiltskin under control in the event that the curse breaks.
When the time is right, Regina adds a new element to the spell - a library, caddy corner to Mr. Gold's pawnshop. She doesn't pick someone to be the librarian - that defeats the library's true purpose in her mind - just leaves the building there, boarded up, broken, and empty. And every day, Mr. Gold is going to see that place, be reminded of the fact that there's no bookworm there to take care of it as there should be, and that's going to be a sort of slow torture in and of itself.
Because in the back of his mind, he's going to think that this bookworm-less library reminds him of something - of someone - and he's going to feel something that Regina knows all too well. For reasons he won't be able to understand, he's going to feel alone.
And that's all that Regina wants for anyone in casting this curse.