I personally doubt that Rick would kill off any of the seven. He writes with his fans in mind, and he's also said that he won't kill off main characters. Now that Frank is praetor, it's unlikely that he'll die. However, it's highly likely that Reyna will either die or step down so that Hazel can rule Camp Jupiter alongside him. I don't think Nico will die, because the entire point of him confronting his sexuality was to tell kids with the same issues in their lives that yes, it does get better, and killing him off would not exactly demonstrate that very well. Whether Octavian dies or not is up for debate. I mean, his death would be great, and all, but I feel like it would be more poetically just if he survived. Most likely, he'll begin the attack on Camp Half-Blood, realize that Gaea is a bigger, more threatening problem, and grudgingly join forces with the Greeks, defeating her. Then, at the ending celebrations, he will be the arrogant auger he is, and there will be that "ah well, some things never change" moment, and things will go back to as normal as they can be after a war. For Leo and Calypso, there are many possible outcomes, the least of which being death. Here is the one I see happening: they save Olympus, Zeus is reminded of his promise to Percy, Calypso is allowed to leave her island IF she agrees to be mortal, she and Leo live happily ever after. (I don't think that Leo will be granted immortality, for a few reasons. One: he'd never be with his mom again. Two: I don't actually thing he'd want to live forever, not as a god. He'd constantly be comparing himself to his dad, or to other gods. It would seriously damage his self-esteem, and then he'd feel bad for beating himself up about it, because he would feel as though he should be grateful instead of self-pitying. Three: remember what Mars said? Life is only precious because it ends.) Last but not least: minor characters, campers, spirits, and the like. As of this, we can only pray it won't be our favorites.