It's not touched upon too much in the films, but Dumbledore's childhood best friend/implied gay lover (was his name Grindlewald?) went after that much power but was stopped about 100 years before Harry's time.
Also, on the subject of no more Harry Potter: I kind of agree. Yes, it was an amazing book franchise and most of the films weren't too bad (I still cannot bring myself to liking the final one), but it's done. It's over. It's a rich world, aye, but by carrying on J.k Rowling would have to top Harry Potter, whilst keeping the books relatively family orientated. A lot of her fanbase are children and teenagers, so she can't make it too complicated or dark due to the children (Hell, Harry Potter arguably got a little too complicated towards the end of the series). Something tells me another series would be just yet another good vs evil series. That worked for Harry Potter, but would it work again in the same universe and have the same appeal?