That's a good idea actually. Make the full next season a prequel to ToMI. That way they could introduce a new villain which you have to beat to get the cutlass (maybe some alter-ego of Guybrush that wants to use the power of the cutlass for something else). In the final episode, you get to beat up that other Guybrush that was referred to in episode 4 (let's pretend that just wasn't a joke). And when our Guybrush wins (he always does in the end), the final cutscene sees Guybrush and Elaine sailing into the sunset, with the intention of finding LeChuck.
Heck, they could pretend that the evil Guybrush actually IS LeChuck's younger brother, and that LeChuck mistook you (the good Guybrush) for him!
Which in turn makes it possible to make evil Guybrush take over for LeChuck, after getting hands on his soul as a zombie or ghost in a deal with the Voodoo lady (always pulling the strings of balance), and devouring it. And LeChuck is gone for good (or at least for a couple of games), and we have a new returning villain.