Maybe give a proper send off to the main Characters in some way. Make it so that they are the emotional part of the next story and something happens with one or maybe all of them dying or ing away etc. That quite honestly would be perfect. I found it weird the way Bill Murrays character just popped in and out randomly in this one. He is the face of the franchise and maybe have his character make the ultimate sacrifice of some sorts by giving his life but have his character very involved in the plot.