Well, you can expect a wide range. Maybe a story where Harry Potter accidentally stumbles upon the Doctor's TARDIS and gets caught up in time-travel shenanigans. Or perhaps a tale where the Doctor helps Harry Potter fight against some magical time-related threat.
There are all sorts of fanfics out there. Some might have the Doctor visiting the wizarding world of Harry Potter, while others could bring Harry and his friends into the Doctor's time-traveling adventures.