The city centre has no site anywhere near big enough, unless the fair was made dramatically smaller.
Having just looked at the size of the Walton Street site, there's very few potential sites that are big enough, I'd say Pickering Park probably looks the best option (though I believe some of the rides need hard standing, so would have to be binned).
Not sure if there is already development but I was wondering about the old Birds Eye site on Hessle High Road and about the old site between Brough BAe and Welton Waters?