Extension switches: -fexistentials

GHC supports dozens of type system extensions, which is great. However, many of them come with their own syntax, which makes separate switches hard to implement. One relatively mild extension is support for existential types. Now, if we adopt the debatable and debated convention that local type variables on the rhs of type declarations are implicitly existentially quantified, then this particular extension does not need any new syntax. Which in turn might make a separate switch such as -fexistentials relatively easy to implement? Cheers, Ralf