A very interesting idea but it might not work the way you would like it to
if players will know where dragon spawns... they will just move the duo lane where its needed... or rotate the lanes once the objective spawns
but what would be a nice twist to your idea would be if toplane acctualy got an objective that could compete with dragon... and then those 2 objectives would spawn and rotate when kiilled... this would also require that the respawn timer would only start once both are dead... so people actualy need to cover both the topside and bottom side of the map
there is a danger to that idea though... since duolane is no longer pinned bot there is no reason why they shouldnt go toplane for a 2v1 lane
riot slept too long on those problems ... now fixing them would require throwing the entire game and its ballance upside down