Comment by jitl

Kotlin is very easily adoptable in existing Java systems and is much safer (although not guaranteed safe)

Zig is fairly easy to adopt in existing C systems and is guaranteed null safe (although not use after free safe)

Rust, although quite safe, bears a fairly high adoption cost as existing code often cannot be ported directly.

Borgo (https://github.com/borgo-lang/borgo) is a nil safe language that compiles to go, so is easily adoptable in existing Go systems.