Jai seems like an overall nice design, conservative and practical. I certainly hope it does well.

If I had to ask for one language feature for game-related code, it would be a compiler backend targeting the various shader "ISAs". It should be relatively easy since the exposed architecture by the APIs are quite high level.

It would be useful, if only to make sharing data layout between the GPU & CPU less annoying (padding float4 slots by hand, sigh ...), as well as constants and other stuff.

Especially as modern rendering techniques are more and more compute shader heavy (tiled/clustered shading etc) we could definitely use some nicer high level language features in there instead of the usual preprocessor contraptions.