WLD, I think you are right,firstly there is no way we could find 5000 Morris Dancers and to train them all to dance it correctly ,in time and a pleasure to watch would be impossible.As has been suggested a single team of Morris,possibly doing a difficult stick dance, a single Rapper side,A display of North West Morris,A molly side,an energetic Cotswold side dancing as "Old Spot style", A Garland Dance team, Switching to 5000 children dancing an English Country Dance that included a Maypole Dance would be my suggested format.All featuring top musicians.
I put that and my hat in the ring for discussion.
Al