I think it would be better to just start with a Turing complete language. I think they should use Lua. It has very simple general data structures that are self explanatory, and its very lightweight and sandboxable.
The only issue is combining config files with other programs. You don't want to strip the comments or formatting when you modify it with another program. Also wish there was a way to specify metadata, like what the possible values for a variable are allowed to be. Or descriptions and help info. With that you could easily convert config files into GUIs.
No! Turing complete config files are even worse than ad-hoc config files.
If your config format is turing complete, you couldn't correctly modify config files automatically. (You wouldn't even know how long until you're done evaluating it.)
If you need more logic, put it in your program or have a plugin system or write a program that generates the config file, but don't put it in the config file.
Many projects start out with just simple config files. But then they realize they need to do logic, and hack that into it. Then they realize they need more complex logic, and hack even more stuff in. And it's just a mess. It would have been much cleaner if they just started out with a scripting language.
Whether you should be putting logic in the config file is a different issue, but as long as people do it or have a need to do it, it's much better than crazy ad hoc solutions.
See these discussions on the issue: https://stackoverflow.com/questions/648246/at-what-point-doe... https://medium.com/@MrJamesFisher/configuration-files-suck-6...
But as soon as scripting is supported, it's impossible to write tools that process the config files and always work, especially with untrusted config files. You can't have both.
So the question is: can code be separated from config (like I've proposed above) (the code can still be inlined in the config file, as long as the "root" of the config file is declarative and the boundaries are well-defined)?
If no, which is more important: parseability or flexibility? It's a tradeoff.
My personal opinion is almost exactly the opposite. If the program's config file requires anything more complicated than a regular language to specify, it's doing too much.