Back in the days, TextMate made sensation with its bundle editor. Anyone could simply update a grammar to fix a highlighting issue, add a missing scope, capture missing symbols, etc. Sure it relied on regexes, which were slow, but the experience for customising grammars (in the large sense) was stellar.
Since the introduction of Tree-sitter in Nova 10, which seems to be a sound technical choice especially in the performance area, creating and maintaining grammars has never been so complex. The steps it requires to update a grammar is worlds appart from what we had back in the days TextMate was around.
The feature proposal would consist of adding a grammar editor to Nova that would take away the need to manually generate and compile a grammar, and simplify setting queries for code highlighting and symbolication.