After MOA Percy and Annabeth "die" and will be in the underworld. Which already sums up the title of the next book being called the House of hades. Do you think Percy and Annabeth will run into old friends while there or in a sudden turn of events will they run into the ghost of Luke Castellan? I miss Luke even though he was a dick but I want to see him return.

I don't think so. They may run into old enemies, but I don't think Luke would have been damned into Tartarus. Don't forget the pit of tartarus is only one part of the entire underworld. It's going to be hard for them, though, because that is where all the monsters and the Titans are imprisoned