Is it possible to add those features? Even at some basic level.
I found this tool that already implemented those features https://github.com/waf/CSharpRepl. It uses https://github.com/waf/PrettyPrompt to do its CLI magic.

It could be an additional flag to load that functionality:

dotnet-script -i --syntax-highlight --code-completion

If there is a possibility of implementing either of those things we could split them into 2 separate issues.