I have a few suggestions of what could happen in Episode 3 or 4. Some of them may be silly, but ain't that what's gonna make it fun? :P
1. Tycho, Gabe and "Carl/Carla" become desperate and go to Hell to get assistance from the Devil. He declines, and Gabe must force him to help by fighting him, alone.
2. Charles returns as half man, half Fruit F*cker.
3. "Carl/Carla" almost becomes a demon because of the Necrovombicon.
4. A battle with the Grim Reaper.
5. T. Kemper (the cat) returns again as an assist character, but in this game his genes have been manipulated by Anne Claire, so he returns as a Tiger-like monster.
6. A 1st-person shooter minigame with Tycho.
7. A hobo ninja, who wants the golden hair of the landlord from episode 1, attacks Tycho, Gabe and "Carl/Carla" frequently throughout the game.
I hope you might consider using some of these suggestions in the next game.