@Raigen: Nigh-omnipotence implies that the person in question has an extensive range of powers affecting things with certain limitations and is some sort of abstract or cosmic being. Z's never been shown to exhibit the wide range of powers that beings like the Choushin or future Franklin Richards have exhibited and Franklin isn't an abstract or cosmic being.
@KingOfAsh: The merging two universes bit was with the mecha Zinv from Dual Parallel Trouble Adventure, not Z.
As for Franklin, what are you referring to specifically? His reality warping is at least above universal level considering he could create pocket universes and even a universe through his reality warping as a kid, so he shouldn't have much difficulty screwing Z up badly here.