Does anything change in Hard Mode other than level restrictions? Because if not, my first run was essentially a Hard Mode game, and I'd have to look into implementing char restrictions instead, for a bigger challenge.
Both ways, I would ban who I consider the big 4... Aya, Byakuren, Suika and Nitori. Aya's going to be the most painful to ban; good as she is against bosses, what I like about her the most is how she trivializes randoms...