During university, I started a project called Treasured. It was meant to be a small project to complete in a few months but the scope quickly got out of hand. Instead of singleplayer, it became local co-op and even later became online co-op. Only this decision made the scope too big. These plans came up before the main gameplay loop was thought of. "Raid temples to fill a museum" was the only idea I had for the game and this did not change after 2 years. Because the channel grew in popularity and more people started to watch videos I uploaded, I felt pressure to create a steam page.
Ultimately, the project was going nowhere since there was no clear design. This resulted in a lack of motivation and thus the end of the project.
Iterations
The first iteration was only the museum hub level. Multiple local players could walk around and interact with things.
The second iteration was about interactions. Buttons, levers, pressure plates and statues to slide, all were experimented with.
The third iteration was all about procedural generation. A level could be made with primitive planes and at runtime it would procedurally be generated. The tiled floor, stone walls, grass, ferns, vines and even trees were all procedural.
A fourth iteration went back to the museum but with networking in mind. Because if a level needs to be selected before it can be played, I can start development. That was the idea.
The fifth iteration was about procedural generation again. It would take another approach than iteration 3, namely replacement. A primitive could be placed in a level and a procedural mesh would replace it for more detail.