In terms of age for the playing character 18+ is fine with me. I believe we can either play the last year students in Hogwarts only, with an invitation for all last years students from other schools. The issue here is smut involving under 18 of age, but it doesn't mean the younger students could not be involved in other adult situations like: violence, killings, dark magic...
The university route is perturbing to me because we'd lose the specificity of each school, the robes, their history in mixing them altogether. If this option is selected, then Gryffindor, Ravenclaw etc...would be simply replaced with Hogwarts House ?
The locations of the rpg could also happen outside Hogwarts as there are many places in HP that are interesting to visit.