On a purely syntactical level, F# has a "light" indentation-based syntax mode which can be toggled on a per-file basis, and OCaml recently got a whole syntax re

On a purely syntactical level, F# has a "light" indentation-based syntax mode which can be toggled on a per-file basis, and OCaml recently got a whole syntax revision with curly braces in the form of ReasonML, which the build system automatically recognizes based on its file extension. Neither of these adds new keywords (ReasonML switches some around, e.g. match is now switch) but maybe one of these approaches might work for language features or breaking syntax changes or revisions.