If two-phase geomorph systems seem a bit constraining, three-phase systems start to seem a bit daunting, and four-phase systems are just a nightmare in complexity waiting to beat your brain into a gibbering heap...what about single phase systems?
If you're thinking to yourself... "Surely you can't have fun with a system of geomorphs where every side is the same?" ...you'd be wrong.
Single phase geomorph systems are arguably the most versatile of all, because it doesn't matter which geomorph edge sits next to which other geomorph edge. No problems with land abruptly meeting sea, no problems with "right banks" being forced to join up with "left banks". They all fit together neatly because they are all the same.
But this means you need to design your complexity within the confines of the single tile.
For example, I developed a goblin labyrinth setting a few years ago. It was comprised of a huge maze spanning continents and mystically plunging into nearby planes. The goblins had lost the ability to keep expanding and the civilisation had collapsed centuries ago. It was a tarot deck, accompanying book, and RPG. The maze dominated the lives of the goblins and shaped their myth.
It would be easy to generate a set of single-phase geomorphs to create fragments of the world in this setting. The world is fairly homogenous in its labyrinthine nature, but incredibly complex at a smaller scale.
Since each geomorph has the same edges, we could use this for a game where spell effects might manipulate the maze. One spell might rotate a geomorph clockwise or widdershins, another spell might swap two geomorphs with each other. Since they all match up, there won't be any cascading effects of complexity.
Another great example of a single-phase system comes from last year's game from Vulpinoid Studios, "Town Guard".
All of the town sectors are on a single geomorph, all of the sectors may be connected to each other because all of their edges are "road" edges. I've toyed with a "pirate" expansion for the game, including so e coastlines for a two-phase system, but the core game has heaps of options with only a single phase.
Other games like "Tsuro" work with single-phase geomorphs too.