If I had to design one of these, I'd go for:
1. Token minimization (which may be circular, I'm sure tokens are selected for these models at least in part based on syntax of popular languages)
2. As many compile time checks as possible (good for humans, even better for machines with limited context)
3. Maximum locality. That is, a feature can largely be written in one file, rather than bits and pieces all over the codebase. Because of how context and attention work. This is the one I don't see much in commercially popular languages. It's more of a declarative thing, "configuration driven development".