I submitted an entry for Spring Thing 2019 and in this thread I want to share some information about the development system I used. It’s called XVAN.
Platforms: Linux, Windows, macOS.
Source code: available under GPLv3 license.
Game binaries are platform independent. What you compile on one platform also plays on the other supported platforms.
Game sources are defined in terms of objects, locations and timers.
It’s documented (English). The documentation includes a tutorial that shows how to build a game from scratch.
Optional library (Starter Kit) with 50+ predefined actions and a dictionary.
Languages currently supported: English and Dutch.
Learning curve: the language syntax is easy to master. What you maybe must get used to is thinking in terms of objects. Each object gets the user input and decides for itself how to handle it (or not). So your story is actually a number of relatively autonomous locations and objects (and timers).
How does XVAN differ from other systems? I think nothing compares to Inform 7. But if for some reason you don’t like Inform 7 and you don’t want to write a game in a lower level programming-like language, you may like XVAN.
How to start? I’d suggest to read the introduction document, then do the tutorial and when you have a good idea how things work, use the Starter Kit and change whatever you want.
Finally, I am working with Strandgames to make an XVAN version that is a back-end to their GUI. This gives the possibility for graphics, sound, on-screen inventory, markup, auto-mapping etc.