I've actually played in an HP rp where it was just simple go to school and intermingle... it got boring very quickly. I like the idea of building a plot. Maybe starting with something simple to begin with and then build from that. 

I love the idea of an antagonist.

I guess the question is do we want to start at the beginning of the year? If so then I would suggest a simple plot to begin with, perhaps something questioning happens at the feast where the students get sorted. Something that would raise possible suspicion. Also the idea that the school could be in danger from an outside force is a lot of fun to play with. 

I found that if you play to get to one significant plot point and then come up with another to get to it keeps things much more interactive for everybody.

ETA: I love the idea of the mascots being objects. I also like the idea of the mythological creatures being used. I'd also like to come up with a basic map for the school if no one would object?

Also I agree that we should be allowed to choose our own age. Maybe say that they need to be in 4th year or higher to keep it fair?