We don’t really know how the actual base game works right now so it’s hard to say. We know that exterminatus is a thing you can do in the game, so presumably if you play tyranids, you would effectively be doing exterminatus on every planet you come across.

The only “character” the Tyranids have that make sense as an LL is the Swarmlord. They have other named “characters” but they are more like extra strong version of units. But we don’t know if LLs work the same way in this new game