I posted my theory already, but here it is:
Guybrush, Morgan and Winslow are in the Manatee right now (also known as "the mouth"). The next chapter will probably start with the Manatee spitting them out by it's lair, an underwater cave or something - possibly beneath (or on) the rock of Gelato - which also happens to be the home of La Esponja Grande!. There they will find De Cava (who may be related to Morgan).
De Cava's story is as follows: he was working on a submarine to take him to the Lair of the Leviathan and La Esponja Grande!, but decided that the idea could never work because there wasn't going to be enough air for him in the sub, so he decided to make the golden animal thingmabobs out of fool's gold and summon the mouth (the manatee, that is) to take him to the lair. What he didn't think about was getting back; he's now stranded. This may be due to the mouth being a one-way deal, or it may be due to the fact that he used fool's gold to summon it.
Luckily, Morgan LaFlay was working with a modified version of the sub co-designed by De Singe in exchange for her skills (or maybe just designed by her), and if she can get the right parts (with help from us, of course), she can make a new one and take everyone back above water, with a big wad of La Esponja Grande! in hand to cure everyone.
Meanwhile... LeChuck and Elaine are out putting the monkeys back in their natural habitat. OR ARE THEY?! Maybe. I'm not sure, I haven't thought into this one much. What I do know, though, is that LeChuck has a hidden agenda, or will by the end of it, and wants to restore himself to his former undead glory, winning back Elaine in the process. Elaine is wise to this, however, and she's given Guybrush her ring as a "deposit" to show that she'll be back.
Upon returning to Flotsam, it becomes apparent that McGillicutty and/or De Singe have framed Guybrush. McGillicutty because he doesn't want the sponge to get out, and De Singe because he wants Guybrush's hot pirate body (and who better to experiment on, in the eyes of the law, than a prisoner - or one's corpse?). Guybrush will be executed promptly, but resurrected as a PIRATE GOD! shortly thereafter, by De Singe (inadvertently?), De Cava, the Voodoo Lady or (maybe) LeChuck. Either he will have to use his new awesome title to defeat the now more powerful than ever LeChuck, or he will have to use it to defeat De Singe (depending on who LeChuck is siding with - he may have resurrected Guybrush in order to get him to rid the world of De Singe, who would be a threat to his voodoo power).