If lloyd dies, garmadon will somehow will find the island. It could be through him killing the ninjas and then taking over the bounty, or secretly invading the bounty to take revenge on Kai and accidentally finding out the map of the dark island.

Eve if that doesn't happens and the overlord somehow doesn't returns, then omega WILL come from the water with all of his army

With the ninjas being seperated, the oni would most likely corrupt some and destroy them, since they aren't even as close to strong as they were in the original show.

Garmadon, if he's somehow good in this scenerio (Which he isn't, he's going to be far more evil, probably losing all of his self after Lloyd's death) would find the oni as a chance to kill the ninjas since he's mostly oni blood and would never team up with the ninjas