Yeah, Nico is captured, leaving a spot of Annabeth to take. I find it ironic that first Percy is the "Dude in Distress" during Lost Hero, and now Nico is the "Distressed Badass". lol, what is with guys getting captured? Perhaps Riordan got bored after Annabeth got snatched in Titan's Curse and decided to switch things up. xD
True. Apollo's "spawn" are likely to be treated more nicely, though, than any child of Poseidon. So many bad blood between Athena and Poseidon, lol. "You turned my favourite girl into a snake-headed gorgon!" "You SLEPT with that very same girl in my ****ing temple, you giant arsewipe!"
Annabeth's addition would add a nice degree of "godly diversity" (nice term, btw) and would actually even out the guy-girl ratio in the group. With Annabeth, you have four guys and three girls, as opposed to five guys and two girls. But hey, even if Nico joins, you'll have four Greeks and three Romans. Camp Half-Blood wins! xD
SoN implied that some powers get recycled, as Frank's situation has proven. The "gifts" are contingent on the favour of the god who gave him in the first place. I suspect that Aphrodite can choose to might her grandchildren Muggles if she were upset with Drew/Piper. xD
PJO fanfiction doesn't require too much Greek knowledge, tbh. The series already set up and extrapolated on a great deal of mythology already; you can simply play within the limits already set by Riordan and be fine. No, I speculate that PJO initially didn't have enough strongly characterised (non-god) characters who weren't Percy, Annabeth, Nico, Luke, Grover and Thalia. HP has bucketloads of minor HP students who have strong, defining characterisation (Ernie Macmillan, Hannah Abbott, Lavender Brown, etc.) The addition of the Romans and Leo/Piper should have rectified that problem, however.