I think it would benefit mostly plugins development. If you take a look at 3 major plugins source:
Database,
REST and
DBIC.
3 different ways to register keywords ( sub anonymous vs ref ). Different version and abstract declarations. die vs croak. Show keyword declarations at top or bottom?
Maybe a style guide on how to decouple business logic from dancer routes (file splitting, modules for business).
But I understand that style guide's are highly opinionated and may not suit every team or project. But a general one would be better than nothing in my opinion. Or maybe a style guide has nothing to do with Dancer2 core...