Risa

I'm pretty sure they already have and just don't know it. Quinn from Voyager has done "everything" and you can't seriously expect me to believe that being part of the collective isn't on that list.

Of course, you could argue that even if he ever had been assimilated, it wasn't real, since he's still Q and the Borg aren't omnipotent. But then again, that's kind of moving the goal post when you consider how many others managed to maintain some kind of independence after being assimilated. Hugh, Janeway, Seven and her internet friends, they all managed to resist the collective in one way or another.

But ok, for the sake of discussion, what happens if the collective does get the full knowledge and power of a Q? Well, it probably depends on how Q powers and the abilities of other such godlike entities in Star Trek work. Is the continuum more powerful than the ascended collective? Does a comparative power level even enter into it or do they have the admin rights to the universe's operating system? Does it matter how many Qs there are, and if so, does the collective count as just one entity or many, and does that help or hurt their chances? Without a clear understanding of what the rules are, we can't really say how things would likely play out, only how we would want to see them played out.

That said, you could argue that we get the beginnings of a unified godlike powers theory early on in TNG. The Traveler explains that the basic building block of all reality is thought. Essentially, Star Trek's entire universe exists in a mindscape, and thought can directly alter reality. With this as our starting point, we can theorize that Q and any other godlike being is directly manipulating a layer of thought that is the foundation of all reality.

That we can't all do this under normal circumstances, but that we can be given access or suddenly do it by accident when we're beyond the edges of our normal cosmic environment implies that something is actively preventing us from just casually hacking the universe. The Q continuum being able to bestow and revoke such powers, even among their own, implies to me that they are the ones policing this stuff.

Now, how a conflict between an unknowable continuum and an ascendant collective plays out is still vague, but I think we have enough point of reference to translate this conflict into a more familiar form. In this scenario, the universe is a holodeck, the Q are running IT, and the Borg are professor Moriarty. When viewed through this lens, the question is, how effective can the Q be at containing a system breach, vs how well can the Borg evade the Continuum's countermeasures? Since we know the Borg are not always the best at thinking outside the box, while the Q do seem to be able to police their own, my money would be on the Borg getting beaten almost immediately (or perhaps even retroactively).

Well, my money would be on the Q, if it weren't for the Voyager crew being able to beat them with muskets. (I don't care if they aren't really muskets, it's still Qs getting killed or wounded by mere mortals, and on their home turf, which those mere mortals can't even properly comprehend.)