Only Croc's backstory would've added stuff, and even then, not sure it would've justified the embarrassing couple of lines he has down at the end.

Kangaroo dropping racist Asian jokes would've been paaaaaainful.

Adding more Joker definitely would not have helped at all. Probably would've just made things feel even more disjointed.

If anything, the movie needed less of pretty much everyone except Deadshot, Harley, and Diablo.