Custom representation for object in the output cell?

To share some of our plans here:

  • There’ll be plenty of ‘userland’ solutions possible already and in the future, like viewof cells

In the works is an implementation of devtoolsFormatters, a semi-obscure but hella cool way to provide custom formatters for object types, and the situation you propose will be perfect for them. See the Immutable devtools formatter for example: we plan to support the same API but to make it available outside of your browser’s devtools and on the page, and in all browsers rather than just Chrome.