It should just be Riggs and Murtaugh and, maybe, a visit to Muraugh's family (since they were in the original.) No Leo, no Rigg's wife (and kids), no Chris Rock. Just Riggs and Murtaugh.
They can't break the trend. They have to bring back Pesci, Russo and Rock. Plus they have to add someone else who will come back for the sixth movie.