- render a list of items, with a panel showing additional details for the selected item.

- update the view when changes from a backend data source arrive, either via some sort of polling mechanism or a persistent connection.

- allow the user to edit fields on the selected item.

- automatically save any pending changes when navigating away from the page.

AKA every CRUD app ever. You are going to need a framework (most commonly MV*) to deal with this in a maintainable way. Whether you use a vendor or create your own, you will end up with a framework.

When you say "vanilla JS", do you mean "just JS with other JS-only libs for special controls, etc.", or "just JS". Because, if the latter is the case, then the challenge is easy:

Virtual list controls that need to manage thousands of items/rows.

Hint: you're going to need a custom scroll bar.

Random things off the top of my head that are a pain without a framework:

- take object of arbitrary depth {a: 1, b: [1, 2, {c: 3}]} and send as querystring parameters to Rails/Node/PHP/whatever backend

- take two parallel ajax calls and run some code when they're both done

- SPA w/ parameterized urls