It has to be as fast as possible, so interpreted languages are out.
You don't want to micro manage memory, so C is out.
You don't want to require programmers to have a degree, so C++ is out.
You want fast startup and not depend on a big runtime, so Java is out.
It has to run on most systems, anything with a C compiler, so D is out.
You want to have fun making something new.
If you remove his last requirement I think lof of languages still qualify. Effeil for instance, and maybe OCaml.
I can understand why Bram wants to target C. Vim runs on almost every operating system and architecture. Right now that's only really possible with C-targeted languages, languages with portable VMs/intepreters written in C, and (to a slightly lesser extent) C++.
If you remove his last requirement I think lof of languages still qualify. Effeil for instance, and maybe OCaml.