Proceeding with the breakdown of the aborted TMGS-II, when Act II ended, the party had just returned from another dimension where they had been pursuing a priestess of Moander. They hadn't caught her, but they had determined the location of the Heart of Moander - the buried ruins of the city of Tsornyl in the Forest of Cormanthor.
Act III: The Heart of Decay
The party could have used one of two portals back home, either the one opened in the lair of the False Prophet or one from the ring they had picked up from the Moanderite leader in Act I. Either way, the party ends up back in the Priory of First Union, the final location of Act I. The trick is to then arrive at the Forest of Cormanthor, and to do that in a timely fashion, they need to use the portal described in my post on Act I. This portal requires rearranging the elven runes such that the portal takes them to the proper locale. Functionally within the game, this serves as a "puzzle" before they could proceed.
Traveling through Cormanthor, the party would battle blighted treants, Bits of Moander, and various undead of a suitable level. Along the way, I was planning on having an elven village that the players would need to ally with in order to find the actual location of the buried city. Securing the cooperation of this village would have proven a challenge and may have led to the addition of an another companion; I was thinking maybe an eldritch knight character.
Eventually, the party would have found the lost city, and this would have been the final dungeon complete with suitable tricks, traps, and encounters. The party eventually would have passed the corpses of the 12 baelnorn who had been guarding the heart along with numerous Moanderites. At this point, a cutscene would have shown the priestess fusing the Moanderite essence with his heart, allowing for the rising of Moander's avatar - essentially a big shambling mound. The avatar would have killed the priestess and the final battle would have been on. Again, the party would have comprised 5-6 characters of about 16th level at this point, so the challenge would have had to be suitable.
After the battle, I would have had a few wrap-up scenes, the last of which would have set up TMGS III. My gut is that the above would have expanded quite a bit to allow for at least a little bit of freedom at certain points, kind of how Waterdeep and the Bastion exterior allowed for a small bit of exploration in the otherwise linear final act of TMGS I. I didn't have any firm plans yet, although the elven village seems an obvious point to do this.
As for TMGS III, the basic premise was that the refugees from Veronsport would have arrived at the Bastion and, realizing St. Cuthbert held no power in Faerun, would have converted to Tyr. The Grand Prelate would have set aside some land for them to build a small village and then appointed the PC the village's lord and spiritual guide. Deacon LaRue would have served as the players' deputy in the religious order and Lord Roberts would have served as the mayor and secular leader under the PC's aegis.
The land would have had a small abandoned temple of one of the evil gods and so I would have made a small interlude where the PC and either Tancred/Verona alone would have cleared the place out. Then TMGS III would have been a stronghold-based campaign that I really hadn't planned too much yet. The stronghold would have been the cleared out temple in case that wasn't clear, and the refugees would have built a small town around it. Some evil deity would have featured, but there were a ton of candidates as yet. Talona kind of holds some interest for me, but I really had no firm ideas.
And that's it! Summarizing the plot only took about two years less than making the module. Next time will be something different... maybe an announcement. We'll see.