I think one of the good things about the self-made adventure game scene is that it allows a variety of different game styles. So in my opinion the next game should be something completely different.
How about
Jessica Plunkenstein and the Dusseldorf Conspiracy? I haven't played it, but it's been getting good reviews (including on
AGers).
For future playthroughs, I nominate the Ben Jordan series. All the (released) games as a whole add up to a reasonably lengthy game.