The whole Shadow Isles thing is just so mysterious and we only know that the Void holds some scary beings based on Kassadin's Lore. So I heard speculations that the Shadow Isles is filled with undead creatures. I haven't seen any interaction between the two. My questions is, would a rivalry between the Shadow Isles and the Void make sense?
I just paired up the two because they are both so mysterious.

The Void is more alien/other dimension, while the Shadow Isles is more dark magic/necromancy

People say that the Void are really scary because they want to or have to potential to wipe everything out of existence but I noticed that we are waaay underestimating the beings that live in Runeterra, I mean Rengar is not from the void and yet he was still able to duel with Kha Zix to a draw, does that mean that if the Void ever does go to a war to destroy the world, the battle is not going to be one-sided and Valoran would actually stand a chance?

well i think that void is much stronger bicouse they are in other dimnsion and who knows what creatures lurk there. Cho is teror but who knows how looks ruler of void.

I see Kha'zix as more of a scout who is meant to go in first and adapt to a world. He's constantly evolving and learning how to be better suited at surviving and killing. So, scouts like Kha'zix would give the Void more information on how to invade a world. I don't think Kha'zix is reporting back or anything, but I believe that the Void is either an actual entity, or has a powerful leader who knows what all the individual Voidborn know.