I settled on a theme for this set of geomorphs – dead ends. No entrance will connect to more than 1 other entrance, and if it does connect to another entrance, it will be on the same side of the geomorph. Like one of those 4-way cap pieces in the various pipe-style games.
Geomorph 16a
Ah, I was just thinking you needed some terminator geomorphs.
Stretch it out to 10 ‘morphs and it’ll be a perfect end to the series.