For Pulp I'd replace Travolta with Beencio Del Toro, for Samuel L Jackson I'd replace him with Wesley Snipes.
I'd give the lead role of Mad Max to Vin Deisel.
If I had to recast Travolta and Jackson I'd be looking at Miles Teller and Michael B. Jordan I think. And for Mad Max I'd love to see Emile Hirsch try that role