Post Big 3 there could be a scenario not dissimilar to the women's Slams where its almost a lottery. Zverev, Thiem, Medvedev, Berretini, Shapovalov and Tsitsipas look the most likely candidates to share GS in the coming years ahead, I don't see that any one of this half dozen particularly stands out above the others. Feasibly, any of them might win three or four GS but beyond that seems difficult to imagine.