Well, Messi is going to spend most of his time in that central space between the lines, might as well make it formal. Actually from what i remember Barcelona looked best this season while Suarez was injured/rested, and Messi deployed as a false nine flanked with two wide players. I guess Griezmann would be tasked with cutting inside from the wing.
I literally have no idea how would they fit Messi, Dembele, Suarez and Griezmann into the starting XI. It would be 2007 all over again, when you had Henry, Messi, Dinho and Eto'o competing for three spots.