Full disclosure: while I have read a fairly in-depth sporking of it, I have avoided the original. What I saw was enough to make me ill as it is.

Do I think that a female Potter could be well-written? Absolutely, although none I've ever seen manage to (see: Holly Potter the woobie). But one of the problems with the Rose Potter series was, I believe, exactly those plot points you find tantalizing. I found they absolutely take focus away from the actual world of Harry Potter by shifting focus to things which make no sense and which in fact often <i>supplant</i> the canon precisely as Sues do (i.e., Let Me Take A False Name As An All-Powerful Druidess So I Can Make The Wizengamut Cower Before Me, Dumbledore being impressed by this, etc.)

The problem with female!Potter stories is that it is almost impossible to do without regurgitating so much of canon that it could get quite boring. I do think it could be done well, and even do so in ways that explore things not shown in canon, like what it means to be a witch in that universe when, while there is not as much overt sexism as Rose Potter claims, there is no reason for it not to parallel the real world to some degree. Frankly there are absolutely gender issues which could be explored--it's even called the Wizarding World, what about the witches?--although if you are male, as your handle suggests, and you want to handle this, please get a woman (and even potentially a British woman as cultures may vary) to consult with on this. The other thing which it could do is explore the little-explored aspects of canon. Harry Potter is known to have plenty of holes, undeveloped parts, and any number of dimensions to explore without adding entire magic systems to an already dubiously developed one. For instance: explore international relations in the Wizarding World more. There might even be ways to work in some of what you liked out of Rose Potter without inventing things that have no place in the canon and which frankly upstage it.

Adding Druids and Necromancers and the like honestly feels to me like adding Lost Elvish Tribes or Unknown Magic Users to Arda, and that almost never ends well. My policy is that in the hands of a sufficiently talented writer, there is almost no Bad Idea (TM) which cannot be deftly handled. Doing so simply takes care, lots of beta-reading, and consideration for the worldbuilding. When you're playing in someone else's sandbox, respecting that still matters.

For some fics, adding whole things like that could easily work if there are outright unexplored areas of the map, of the magic, etc. One of my favorite canons I default to absolutely has this, because of the directions canon can grow, particularly post-plot. Harry Potter doesn't seem to have that much unexplored, at least not in the immediate region of Britain. Working new groups in from Africa, Mesoamerica, Oceania, India, or other places we just don't know much about could do this. Locally, however, things are fairly complete.

If you think it's so incomplete that you need to add entire things with your own rules and history and interactions, honestly? Try making it original. Develop it into something no one's seen before, and make it fantastic. Personally, I would rather see the results of that than Harry Potter turned into something it isn't.