Support a more common syntax definition format
GtkSourceView uses a custom format for syntax definitions that is not used by any non-GtkSourceView based text editors. GtkSourceView also includes syntax definitions for an extensive set of languages. However, maintaining such an extensive set of syntax definitions is a lot of work and new language definitions are only added when someone from the community is willing to help maintain it.
Since there are other open source text editors and code related tools out there that also want to support syntax highlighting for an extensive set of languages, they are also maintaining their own sets of syntax definitions. Some of these tools have far more users, more contributors and more funding awailable than GtkSourceView.
From this point of view it would make sense for GtkSourceView to support a syntax definition format that is shared with other open source editors in order to share the maintenance work.
One of the most widespread syntax definition formats are TextMate Language Grammars. Besides TextMate itself, GitHub uses TextMate Language Grammars to highlight source code across their platform. The collection of grammars they use can be found here. TextMate Language Grammars are also supported by Visual Studio Code, Sublime Text and Atom, three widely used text editors.
Sublime Text also has its own syntax definition format which is based on TextMate Language Grammars. Their syntax definitions are freely awailable and actively maintained.
Then there is also Tree-sitter which is a new language definition format developed by GitHub which already has definitions available for many languages. Atom supports Tree-sitter definitions for syntax highlighting.