So my players prefer higher level campaigning than what most APs get to. I've decided to merge Shattered star into my S&S campaign to be able to provide a 1-22/23 campaign. In the Spoilers below I'll point out what sections I've merged where to see how you all feel it flows. I used the slow path to keep leveling where I wanted it, and used fiat bonus xp when it was needed to get them where to appropriate level. (Note my players all were given the Leadership feat for free and their cohort was the first mate on there respective ships since they had a ship a piece and 4 or 5 mule vessels in their fleet after book 2) We've gotten level 13 and are about to face an old foe.
Sorry it's such a wall'o'text. Let me know what you think.
They then moved to Raider's of the Fever Sea and I simply upped the plunder to allow them to head to Port Peril a bit early so they wouldn't out level the next few sections.
After the Free captain's test in Tempest Rising, I had a priest of Abadar contact them for assistance, as they could do things the church couldn't, to stop the events from the Module the Swords of Sin I had the Pc's start in the dungeon and flattened the starting and ending cr's a bit to provide an 8-9 experience. I placed a teleporter at the end of the module to the 2nd level of the lady's light and the lust shard there to merge into SS.
They were then called back to begin At A Lady's request. Between the two leads I had the characters start the Asylum Stone by beginning up a rive. They then came back and finished the second lead before continuong to City of Strangers. Finally they returned to the Shackles for the Regatta.
They jumped in the Island of empty eyes and did both a prize like no other and the abandoned fort before heading to The undercity and the black keep. After gathering the gluttony shard they moved into the ruins of Ghol gur. They followed that with Feast of Spoils Before heading to Start News of Destruction and Windsong Ruins.
Here I created a fiat stopping point by having a arcane gateway spring up that a Mcguffin Npc would have to disable taking a fair bit of time. The pc's completed Blood in the Water and Islands of the Damned before being called back for Down into Doomsday and the envy shard it contained. They then went to the Black Tower and hit 13 right before the end. We are currently finishing Giants of Wrath and moving to Guiltspur depths, where they should hit or be very close to lvl 14, after which we're headed to Harrigan must die to kill him and also take the greed shard from him. they are then going to finish Chelish Armada Storming fort Hazard and Caves below Lucrehold (taking the Pride shard from Bonefist). Tessa Fairwind will begrudgingly take the Hurricane crown for a time to allow the group to finish Shattered Star.
There are no major changes from here until the final fight with the Reliquary. After that fight a portal to the moon opens and we begin the Moonscar module.
Izmiara has sent the 4 sisters of vigilance through the somnul gate ( changed from lvl4 ranger succubi to a 6 ranger/2 fighter 8 racial hd succubus, a 9 cavalier succubus, a 9 inquisitor succubus, and a 7 barb/2 fighter succubus with 15 hd shantak mounts) they make an opening for a few of Izmiara's other lackeys to grab Xin's contraption and corpse the pc's then move through this module which I've bumped to cr 19-22.
At the very end I have an approximate cr 26 fight in which a Xin device must be defeated as he is trying to capture a time dragon to return him to the past to fix these mistakes. He goes through phases using 7 forms that have their own special attacks as well as various boons and banes.
Upon defeating Xin the final time. The time dragon will offer to take them to any time they desire for saving him. if they accept a time distortion will leave them in a fight with Cthulu but with strange new (read:Mythic) powers that should make for an honorable TPK or a well-earned victory, either way it will be a legendary ending.
From a regional/thematic perspective, merging Skull & Shackles with Serpent's Skull might be a bit easier. They could even start out in control of the Free Captains faction for Serpent's Skull (instead of Kassata Lewynn), depending on when you start interweaving the plotlines.