Hallucinate just the missing feature you need.

Not recreating a whole app.

When you have an extension showing it in a sidebar on the side, it doesn't need to be a whole new app to use alone (that's very hard), it can just be a single feature that's missing just for you.

Adding one feature to Gmail is much easier than reinventing all of Gmail.

But today you can only do the latter, so it doesn't happen unless you have a really killer feature, enough to make the whole "reinvent all of Gmail to get you to use that instead"

Gmail Filters ++++ but with turing complete code that auto-assembles according to my high-level intention could be amazing.