Will Theon Be Able to Rescue His Sister?
Now that Theon has basically reclaimed his status as a Greyjoy, it's clear he's going to try to defeat Euron and save Yara. If Euron is sailing for Essos, though, it will absolutely be a fool's errand. Theon has no way of knowing where Euron is going. Even if he figures it out by some miracle, we highly doubt he'd be able to catch up.
Now, if Euron is going to the Iron Islands (maybe to wait out the Winter or maybe to gather supplies), there's a chance Theon could catch up. Obviously, we know Theon is heading home. We're just not sure what he'll find once he gets there.