Player-types: Shoo! This would be a very boring discussion to you!
I plan to run this AP soon from parts 2-?, and I think in order to maintain player interest I need to create more of an overarching theme. So tell me which of these ideas strikes you as the best overall option.
1. At the start of part 2, have M'deggog already manipulating events. Alter his motivations: he's lost, trying to find his way back to Ilmurea, and to his frustration nobody knows where Saventh-Yhi is. So he's behind one (or more) of the factions and, up until the city is discovered by his faction, might even be supporting the PCs' efforts to get there. Throughout Part 3 he's hoping one of the factions will find the door back below. Once Juliver turns up, he'll use the Maka-Yika to try to abduct her so he can learn the route from her. Here the theme would be devourers vs. serpentfolk with the devourers continuing to play an important role in Part 6.
2. Alter Vyr-Azul's motivations. Until early in Part 3 he has no clue how to re-unite his deity's parts; but when the PCs activate a Spear, he senses it. With their old scrying lens broken he's reduced to sending serpentfolk up top by magic; they plant evidence that leads the PCs to accumulate discoveries that he can use in completing the ritual. Once he realizes the PCs might inadvertently destroy Ilmurea the race is on. In this case the theme is that of a race that the PCs only slowly realize they're engaged in.
3. The Coils introduced in Part 6 take on a role much earlier - from Kalabuto on, all the factions are joined by 'helpful native guides' whose actual intent is to encourage a widespread slaughter of the various surface tribes. Once the PCs enter Part 4, any 'helpers' in their faction would do what they could to sabotage efforts to breach the Vaults, and probably attack all the expedition camps "under false colors" to pit the surface factions against each other. Here the main theme would be determining who's behind all the sabotage.
I like a lot of the flavor of the AP, but obviously I feel like the villain needs more reason to be the villain, and the heroes need to be more "on the spot". I'm not a big fan of "rescue Eando" as a motivation, because my players tend not to care about NPCs they've never met, however famous they may be in-world.
