What I'd really like is a tsc that creates code to check types at runtime, like for API boundaries and parsing unknown input. Kind of like a built-in Zod. Maybe it's just an automatic type guard anywhere you have an "as SomeType" or an ignore directive.
You can use io-ts [0] to define your types, and it'll generate functions to typecheck for you. Syntactically it's a bit gnarly and the documentation isn't great; a first-party solution would definitely be nicer. But it works, and it's amazing that it works.