Isn’t that basically what IEx.pry() does? Adding it to code causes the pry debugger interface to pop up in iex. Without IEX the BEAM doesn’t have a normal input/output setup so another interface would have to be made from scratch, which with the BEAM’s multi-core nature would be very… interestingly difficult to work well, so it’s always best just to use the built in shells. 
Yep, unlike Ruby the BEAM is fully multi-core and parallel so doing a shell without a synchronization and overriding point is rather amazingly difficult (even Elixir’s shell is built on Erlang’s due to the difficulty to get it right, that’s why a couple of the limitations of the IEx shell exist).
EDIT: You could indeed make such an interface, you should build something on top of erlang’s shell if you do though, but it will still be a surprisingly amazing amount of work to do.