I'm going to go against the grain and say forget the 'fun'. People call Crackdown fun, I say they have a different definition of fun to me.
Character customisation is irrelevant, instead build on the characterisation and storyline, why isn't it possible to make a compelling storyline in an open world?
I did love GTA IV, by far and away more than any of the previous games, but I think 5 should be the end of the series. It carries too much weight of expectation and controversy. Make a final swansong, all 3 epic cities at the standard of Liberty City in IV, an equally epic storyline and put the series to be.
Seconded for a sequel to Canis Canem Edit (Bully) by the way, that was a very good game.