If Rosalina got her own franchise that had a focus on story and her character, and they somehow got a love interest for her there that works and adds to her character, then I would absolutely be fine with that. Having a love interest doesn't mean she has to be the damsel in distress or something. It can be the other way around, or nothing of that kind can even occur, and she simply loves someone and they live together and maybe protect the universe together. Or imagine a story involving someone she falls in love with but they fall gravely ill or something and she puts her maternal skills to the ultimate test in order to make them healthy again. They could also introduce the whole dilemma of "I am falling in love but that's a problem because I'm immortal and will outlive anyone I fall in love with", and how that effects her (I could see that being what ends up making her so monotone).
There are many interesting narrative possibilities regarding Rosalina and romantic feelings that could be explored, so I am not against the idea of her having a love interest as long as it is interesting, makes sense, and actually adds more depth to Rosalina. However, doing something like turning her into a damsel to be saved by Waluigi (I know that picture is a joke but I may as well use it as an example) out of nowhere, would be a huge no-no and would crap all over everything they've built up so far with her character. If Rosalina is to be paired with someone, I want her to be portrayed as the dominant one or equal to her partner, assuming we are taking Rosalina in her current, post-Galaxy form. I'd be willing to let something more submissive on her end play out if it was depicting a younger Rosalina though, before she became the wise, confident, tough, and awesomely powerful entity she is now. That could give us more insight into how she developed as a person.
I also don't think Rosalina's partner necessarily has to be male, by the way, if she ever gets one. Nintendo is doing gay relationships with Fire Emblem: Fates. And they've had stuff like transexual/transgender characters in Mario before. So if they gave Rosalina her own franchise and depicted a lesbian fling she had at some point or something, then I'd totally be fine with that as long as it was done well. Same with just pairing her with a guy.
However, pretty much none of those things are gonna happen. As things are now, I wouldn't want a canon relationship for her. Within the current Mario universe, there is nobody I can see them making work (I would say Bowser if it weren't for the fact he has to be forced as "DURRR I'M EVIL AND I'M GONNA SEX YA AGAINST UR WILL" every now and then, and if he were human), and any pairing they come up with would probably be forced rather than naturally developed. And in all likelihood, I COULD see Miyamoto being a **** and just deciding to make her a damsel for some male character, assuming he can just do whatever with her even if Koizumi disagrees. The LuigixRosalina ship teasing in Galaxy was cute, but it makes things confusing because Daisy is supposed to be Luigi's girl according to Nintendo, but he frequently shows affection for Peach and then he had that crush on Rosalina in Galaxy, so it's like he can't make up his mind.
I would be OK with them doing some ship teasing with Bowser and Rosalina, or even someone else who might make sense, in a Paper Mario game or something, though. Rosalina doesn't even have to reciprocate, it could just be one sided with Bowser getting flustered around her or something. Maybe he likes her in part because he's amazed by her power. If Rosalina did get into Paper Mario, I would want them to go full meta with her god powers and have fun with that, and Bowser crushing on her in part because of those powers, could factor into that pretty nicely.
ramble ramble ramble :V