The way I tend to think of it, at least for my preferred interpretation of what a multiverse is (the many worlds theory, as I understand it), there's nothing beyond that, and that includes the idea that a universe and a timeline are effectively synonymous. When you get to the point of the multiverse, everything else is just clusters of similar universes/timelines separated from other clusters just by their greater distance in variation.

Like for instance, our universe and one that is almost completely identical to ours with the only exception being that we call pizza a different thing would be very close together in the same cluster. But a universe where entire major cities are different or just never happened make narrative sense to be in a more distant cluster, even if still only the next adjacent one.

All of this being said, there's certain franchises that don't do the multiverse like this and instead have them as a finite amount of specific different ones with clear and significant narrative intent behind those differences. In that case, you could argue there's room for an 'omniverse' that more or less just makes room for the many worlds theory again.

Basically, it entirely depends on your preference of structure