Sure, anything can happen in the future; but you're speculating about relationships that don't actually exist in the MCU, and you're banking on onscreen chemistry that remains absolutely unproven.
It just makes more sense to water the seeds of a relationship that *has* been planted, where the chemistry has already been shown to exist. Besides, bad girl-bad boy (as well as good girl-good boy) makes for a boring romance. Natasha's been dealing with cold-blooded killers all her life, and probably even dated more than a few. And none of them were ever right for her, obviously. I'd rather see her hook up with a knight in shining armor....the kind of guy who'll give her the respect and the security and the redemption she so richly deserves. There's only one guy who can give her all that.
You realize that Bucky and Natasha together are literally the sappiest, most loving, romantic, adorably happy couple together, though? They spend all their time cuddling, forehead touching, dancing in the rain in front of the Eiffel Tower, twirling on rooftops in the snow, leaving roses for each other on their pilows, and just all around being ridiculously affectionate and content. The thing that makes them work is that because they are so comfortable with each other and see each other so clearly, they let their hard exteriors fall away and are completely themselves with each other. The notion that he wouldn't be exactly the kind of man she wants and deserves is laughable to me.
Case in point:
And honestly from the way some people talk about these movies, you'd think they have absolutely no imagination regarding what could come in the future? Like... a few years ago when we'd only ever seen Natasha in IM2 and Steve in Cap 1, if someone had said "oh I think those two could become friends or more" you'd have a chorus of people replying, "Ehh, I don't see it, they've never even talked." No one was even thinking about Steve/Nat seriously in Avengers either, everyone was certain they were going to give her Clint as a future love interest. A lot can change after just one movie. (and again, there's really no basis in the fear of Scarlett and Sebastian not having chemistry alright - two attractive actors who are that talented and love their roles as much as they do are absolutely going to have chemistry if they play love interests - and neither of them have failed to have chemistry with their love interests in past roles)
And lastly, consider also that she has this issue where she feels like she needs to live up to expectations when she's around Steve? She has to live up to an ideal because it means the world to her if someone like Steve accepts and respects her, after the life she's lived. Remember that face she made when Steve got angry with her on the Lemurian Star? She was beating herself up realizing she'd disappointed him. That makes for a solid element in a friendship with tones of brother/sister to it, it doesn't really lend itself to a romantic relationship. No matter how much Steve accepts her, and Natasha knowing now that he accepts her, she is still always going to feel that way, that she owes him, that she needs to be better around him.
Ergo, that's why someone like Bucky would work so well for her, but whatevaaaa.