I'm really not a fan of crossovers because I feel like most series are contradictory in some way. Like I don't think HP is easy to cross with other fantasy/magical series at all, because everything has it's own magical system. Back in the day when I was super into Vampire Chronicles, people used to cross it with other vampire stuff, especially Buffy, but that was just terrible because Buffy vampires are totally different from Anne Rice vampires.