Author has written 1 story for Harry Potter, and Inheritance Cycle.
I'm a 27 year old software engineer, avid reader and staunch horror and fantasy enthusiast. I am a man of eclectic tastes but I mostly stick to Harry Potter fanfiction as I feel the world has a lot to offer and oddly enough, the amount of shoddy writing Mrs. Rowling worked into her last books creates a veritable motherlode of potential hooks with which to work with the characters and their story. You don't even need to work with the main characters in the story... the world that was created for Potter to live in is rich enough that you can craft almost anything you like and make it work with some effort and creativity.
Granted, everyone's mileage may vary and we all choose to tweak the aspects we like least of Potter's world but, in the end, the back story and setting are great ideas and the systems that govern the world are generic enough that we can remove, change or improve on them seamlessly to create stories that appeal to almost any audience. Let's face it, magic in Potter's world is flexible enough that I'd expect things like Apocalypsis from the Sky every so often and instead the maximum exponent of destructive magic in that world is a beam of green light that kills only one person and can be blocked by anything that is not the target. For me, this presents a great many opportunities to create almost anything I can imagine while avoiding the very daunting task of creating a world of my own. I keep a setting and characters I find awesome and exercise my creativity where I want... the way I see it, I get to have my cake and eat it :P
If I had to recommend a wizard's tale that can set the tone for what a Wizard (note the capitalization) or the denizens of the supernatural should be I'd look to Mr. Jim Butcher and his Dresden Files series. He's a master story teller who has created a world that feels supernatural without sacrificing the veneer of normality in which we live, so much so, that you can look out your window at night and think that the things that he relates could very well be true. His supernaturals feel that way when they have to and like normal people the rest of the time... that is much harder than it sounds. If I had to criticize something about him though, is that his main characters tend to have a VERY effective plot armor which makes for some very interesting acrobatics with the storyline to keep them all in one piece (although it could be argued that it makes it all the more impressive when someone dies). It is par for the course in the fantasy genre though, so I can very much live with it. If, instead, your tastes run more to a grittier, more down to earth approach to battle and politics; if not magic, check out Mr. G.R.R. Martin's work in the Song of Ice and Fire series. Don't take my word for it, give their work a chance and I'm pretty sure you won't regret it.
There it is. While the way I write and think of my stories has been influenced by a lot of things, I'd say that those two authors mentioned above are what has most shaped my style such as it is -and Tolkien to a dergree, but I imagine this is true of anyone who has ever put pen to paper and tried to tell a fantasy story-. I think that a healthy mix of the works of those two authors are what a fantasy story ought to be like, so if you're in any way familiar with their work you'll know what sort of things to expect from me. It should be noted though, that while I do try to emulate such works I don't really think I'm anywhere close to their caliber :P Still, practice is what's needed and feedback goes a long, long way to help improvement.
Lastly kudos to all the people at DLP (that is dark lord potter .net) for their excelent work stockpiling the best that the world of fanfiction has to offer. Those guys are responsible for many an almost missed deadline :P Go check their forum. You're certain to find something you'll like.