I actually would prefer if Harry, Hermione, and Ron are not in the expansion.
What I most admire about J.K's writing is her ability to create an alternate world that is populated with characters that aren't only credible, but seem to live and breath even in the margins. Even when tangential characters like Cho Chang or Garrick Ollivander aren't present in a particular scene or even in an entire book, as a reader, I always felt sure that they were there in the periphery, living their lives and going about their magical business.
We already get to visit with the trio in Hogsmeade, so I think it would be grand if we get to encounter other characters in Diagon Alley.