I don't know the answer, but I suspect it's not one-size-fits-all.
Some "modes of operation" might be clearly defined, such as: output is Scala text, or terse output as currently. In former mode, "strings", latterly, "strings".toString without quotes.
I've often wanted a verbose beginner mode, with something like scala-clippy enabled.
I think the REPL should enable various interactions with the language: evaluating exprs, of course, but also making it easy to inspect byte code and detect how syntax is desugared or how certain constructs are compiled (lambdas, singletons, lazy vals, etc).
The workflow is obviously different for a newbie and a power user. Speaking of which, current "power mode" is probably underused. So the interface should invite users to explore increasingly powerful features of both the REPL and the language.
A useful exercise would be to do this for spark-shell users.