Theoretically, with a lot of time, talent, and care, it <i>might</i> be possible. The biggest problem with genderbent!Harry is the degree to which plagiarism due to simple find-replace all on the text might happen.

Also, I am much less skeptical of genderbent!Harry working  than I am of actually turning Rose Potter herself into less than a horrifying mess. To be frank, I remain dubious, given that the point of anything added by her that wasn't just 'and she's female' was about upstaging canon in highly illogical ways.

If you really want to do this, get more than one beta--and make sure one of them is going to absolutely eviscerate the details, because you're already trying to work with material that is one of the worse fanfic out there. As authors, we get super-attached to our work. Something like this, you'll need someone both able to accept the AU elements and still willing to wield a hammer at it if it veers off into uncertain territory.

On the subject of the US Wizarding Culture: Despite the knee-jerk reaction both in my using and in how anyone should react to my using the phrase 'Beware Eurocentrism,'  do legitimately keep in mind that for all the cultural differences, and for all the ways it is very much a melting pot in ways Britain can't be (for instance, not having a border with Mexico), the <i>dominant</i> culture of the United States is inherited from a lot of European heritages. Notice places I mentioned: Africa, India, Mesoamerica, Oceania--add in East Asia, South America... basically anywhere which has significant cultural influence that <i>isn't</i> inherited from Europe. In the US, you could get a fascinating view of what happens when various cultures try to fit together, because there are so many of them--and there are definitely differences between US and European culture. But there are also fundamental similarities.

In particular, if you want to use some of the things you mentioned liking in Rose Potter--Druids and Necromancers--try to come up with plausible origins. Neodruidry is a real thing in our mundane world, so maybe there's hints in the Wizarding World, but figure out how and why. Also figure out in what culture Necromancy might flourish, because otherwise it's Designated Evil and that gets a bit boring.  Try something a little new and off the beaten path. For all that Druids and Necromancers were not in Harry Potter, frankly the concept of them really isn't novel as far as storytelling goes.

In general, I think my point is that if this is an exercise in writing, do the exercise right or you'll learn all the wrong lessons from it. Quick exercises can be very useful and a good way to get used to cranking out copy (the kind of five-minute exercises in some writing classes, or 24 Hour Comics) but in general I'd expect them to either need severe editing or not be terribly polished--which is fine if that's what you're going for. If you're aiming for polish, though, do research and worldbuilding into things Rowling didn't cover.

Also, for the love of all things good and Canon, if you do try to write a genderbent Harry, make her <i>actually</i> a genderbent Harry and not a psychopathic lunatic.