Well I guess Im a little late when it came to reading mark of athena, but trust me, Im like the biggest Percy Jackson fan, ad the ending for MoA literally killed me.
BUT HERE ARE MY PREDICTIONSSSSSSSS:
Percy will meet everyone who died in the first season such as Luke and theyll all help him and Annabeth storm the gates.
Annabeth will die I reckon in the 4th book, and Percy will get pretty angry and destroy everyone in the final book.
Zhang will make the decision to pull percy back from tatarus as one person needs to shut the door on each end (which may be annabeth on the tartarus end.)
Reyna and Leo get together