Currently it's possible (and pretty easy!) to create custom terrain stamps and use them in your playfields. Unless I'm missing something though, it's not possible to include them seamlessly into your scenario so other players can subscribe to a scenario and play it without additional work on their part copy-pasting the stamp pictures and overwriting their existing stamp database, which of course adds its own drawbacks. Would it be possible to have the game utilize custom stamp databases and images that are included with a scenario, and allow the .raw or .png images necessary to be included with a scenario when publishing them on the Workshop? This could also open up a new area of people sharing their custom stamps and stamp definitions for use by others. While the new terrain system is amazing (seriously, I love it), there are some ideas for randomly generated planets that I can't realize with the current terrain shapes currently available in the game. Opening up the ability to create our own stamps and include them in a scenario would let people create some very unique and cool looking planets.
We're still waiting... I hope at some point that scenarios will contain all required files and folders, to make it easier to share and to avoid conflicts with the default game's settings. Quite frustrating to have an update/ verify files after bugs that erase half of the stampdatabase.txt file and only realising it when loading a customized playfield...