I’d really love this. I am thinking about commands like cabal add <dependency> or cabal auto-modules <folder> which modify your .cabal file, simplifying current workflow. I guess these subcommands might be difficult to implement if you contribute to the cabal-install source code, but way more easy (and less burocratic) if you show a poc build by your own.
As someone said in the github discussion, a complementary subcomand like cabal metadata which prints useful data structures to the stdout, might be necessary so every external subcommand has a common initial point .