FH is already very thin and tall, probably not practical to stick another stage on top even if it could work payload wise. Besides that, SLS and New Glenn are t

FH is already very thin and tall, probably not practical to stick another stage on top even if it could work payload wise. Besides that, SLS and New Glenn are the only ones with comparable lift capacities, SLS is way too expensive for the capability and the massive SRBs make the ride pretty rough, New Glenn is probably a feasible alternative, although probably more expensive than Starship.