Ooh I like that question. It’s a tough question for sure, but it’s a great question.
It took me a bit to answer because the way I view this show has changed quite a bit over time. Mostly in the sense that I’m starting to trust the writers more and more. Because that’s the kind of story RWBY is, as it goes on I can see just how many things which I hadn’t considered important, were entirely on purpose. On that note, it’s also been a very strange thing to look back on Ruby and Penny’s interactions and wonder “oh, was that on purpose?”
To get back to your question though. If someone had told me a few weeks after volume 7 ended that Ruby would end up in a romantic relationship by the end of the series, I probably would have reached this conclusion: I hope it’s penny, but I expect it’s Jaune.
That’s the tl;dr version, the longer answer…
First of all, I don’t think you were wrong to read Ruby and Penny’s interactions as romantic. I already liked the ship before volume 8 and part of that is that it’s easy to see their interactions through that lens. The fact I never considered canon Nuts and Dolts an option before is because there’s always been an alternative explanation.
They’re best friends.
And the best friend category can be (and has been, in other shows) used to explain a lot. To the point where writers will have other characters mistake them for a couple but you just know the writers will never consider it as anything other than a ridiculous joke.
And of course, I have more trust in the rwby writers than that, but the interactions between Ruby and Penny never really went beyond what could be (easily) explained as something best friends would do. And that’s not meant to be dismissive either, it’s just that I’ve seen this before. Two female characters who are Best Friends, and by the end of the show the writers have hammered it in again and again and again that they could never be in a romantic relationship.
It’s very hard to consider the possibility that Nuts and Dolts will be an exception to that.
Volume 8 makes it easier. Best friends still works but… it’s a bit harder to be convinced by that. It’s a little easier to see the possibility that they’ll develop this into a romance. I honestly have no clue if the CRWBY will go for it, but volume 8 has led me to believe that they can. If they want to. It seems possible, for once, that they want to.
Before volume 8 though, I considered it a nice idea. If the writers went that route I would have liked it, but actually considering it a possibility? I couldn’t trust them with that, not yet.
So that’s why I would have hoped for Penny, while also completely writing her off as an option.
Why I would have picked Jaune as the most likely option is really just process of elimination. I mean, what are the options even? I’d assume whoever it is would have already been introduced by volume 7. Taking into account that Renora and Bumbleby are both canon, that leaves very few characters for Ruby to end up in a relationship with.
In fact, once you leave out Penny, the only people Ruby’s close with are Weiss, Oscar, and Jaune. I don’t consider Weiss and Oscar options now, and I wouldn’t have before volume 8 either. So all that’s left is Jaune. He’s nice enough, and he’s really grown a lot from where he started. It’s not impossible for him to end up with Ruby. And before volume 8, that would have been enough for Jaune to be my best guess.
I will say though, if we assumed for a moment that f/f ships and f/m ships had no difference in their likelihood of becoming canon? The only time Penny wouldn’t have been my guess, was when she was presumed dead for three volumes.