Having thought a bit more about it, I'd say we can start with named parameters only (and add positional parameters later on if we like).

The existing named parameter syntax doesn't work for MongoDB, though. But whatever we invent, there may be a conflict with the query languages of other stores we support in the future. So I'd use the existing :name syntax by default but allow grid dialects to configure another character for denoting named parameters. E.g. the MongoDB dialect could use %name instead.