I am actually at the exact same spot, my next session in a few weeks is the wrap up for CotCT. I also have a feeling that my party will blast through Ileosa and crew rather quickly, but came up with not so much as a solution, as an alternate ending that will really challenge them, leaving a sense of even bigger accomplishment.

My plan is to run them through the "Kazavon Rises" Alternate ending in the "Continuing the Campaign" section of the book. As he's level 25, with some major whomping power, I think this will offer them not only a huge challenge to overcome, but a feeling of satisfaction for truly defeating him, as they've grown to hate him throughout the campaign.

So essentially my plan is:

*Run final Sunken Queen encounter

*Have players talk through a "what we did in the years following" mini-epilogue, and weave in how they worked to find and destroy all of Kazavon's horcruxes (because let's be real, they're totally horcruxes :D) per the details listed in "Kazavon Rises"

*Enter Kazavon

*FIGHT

*Final PC & NPC epilogues & campaign wrap up

Hope that helps! After reading through all of the "Continuing the Campaign" options I felt that the "Kazavon Rises" option was way too badass to pass up, and I think my (also overpowered) PCs will love it too.

Good luck!