Your idea of complexity is totally backwards. Swift has some types that do different things. Maybe you could add features, thought out to a vague level, that infect every part of the type system, and require redesigning major parts of how the language works, so you can define a few less types! C/C++ have const [which doesn't work well], so surely you can just throw such a notion into Swift's type system! Well that's not gonna make the language simpler.

Go make a statically typed language with Mutable<>, in some form. Then talk.