Madness is designed for 5th-level characters, so your party should be at or near 6th level by the end of the Trilogy. There are several choices for where to go next:
- Cut-Throat's Gold (in Tales of Freeport), 4th-7th: Following a treasure map. Written for v.3.0, so will need some converting.
- The Soul of the Serpent (in Tales, v.3.0), 5th-7th: Serpent people activity in the wake of the Trilogy.
- Gangs of Freeport (Adamant Entertainment), 6th: A powerful new gang arises. (IMO, this one is seriously flawed because it underestimates how powerful Finn's Syndicate and the Canting Crew really are.)
- Blood of Freeport (Adamant), 6th-8th: Strife between two merchant families. Heavy on intrigue and social skills.
- Shadows in Freeport (Goodman Games), 6th-8th: A nasty haunted house scenario.
- Peril in Freeport (Paizo), 6th-8th: A sea-faring adventure. For Pathfinder, so will need converting.
- Fair Salvage (in Tales, v.3.0), 7th-9th: A mysterious ship appears near the Serpent's Teeth.
- Vengeance in Freeport (Adamant), 7th-9th: Milton Drac left behind a few surprises when he died.
- Crisis in Freeport, 7th-9th: Resolves the Succession Crisis, and sets the stage for the Pirate's Guide era. Plenty of politics and intrigue.
- Dark Wings Over Freeport, 9th: Crime boss Finn requests the PCs' help.
- Hell in Freeport, 10th: Descend into Hell to foil a diabolical villain. For v.3.0, so will need converting. Be aware that some devils changed CR in v.3.5, so some encounters (particularly the first one!) will be more difficult. See The Book of Fiends for v.3.5 updates of most of the new monsters.
Alternately, just dive right into this mega-adventure after the Trilogy:
- Black Sails Over Freeport, 6th+: Defend Freeport from a cult dedicated to a forgotten evil god. Designed for characters to start at 6th level, and will advance them at least 5-6 levels, possibly into the low to mid-teens. Much of the adventure takes place in another dimension.
Black Sails is the highest-level adventure ever written for Freeport, so you'll be pretty much on your own after that. Freeport wasn't really designed with epic-level characters in mind--there are only two 20th-level NPCs in the entire line, and none of epic level. However, some of the adventure ideas in Cults of Freeport and Buccaneers of Freeport are suitable for high-level characters, particularly those for the Order of Starry Wisdom and Obsidian Brotherhood (Cults) and Alfhild and Scevola Hest (Buccaneers), which involve NPCs of CR 15+.