You can absolutely have fancy UI elements that provide additional functionality. Most OSes don't have built-in 3D visualization widgets but that doesn't mean you can't write CAD software for them.

My point is that your custom widgets should make an effort to look and feel as much like native widgets as possible. Any skills the user has in using native widgets should carry over to your custom ones. So your custom text field should look and behave like a native one until the user types two left brackets. When they do, the menu that pops up should be a native menu or one designed to resemble one very closely.

Thanks to web-first development and lazy cross-platform UIs, standards in this regard have deteriorated to near-nothingness. Buttons don't have to look or even behave like anything else on any platform. It's perfectly reasonable to expect the user to relearn the UI for any application. Modern UIs spiritually follow in the footsteps of Bryce 3D rather than any Human Interface Guidelines. And that peeves me.

For all their faults, Apple got Mac users to have very high standards in this regard for quite some time, which led to a bevy of good-looking and approachable applications, at least until post-skeuomorphic macOS took care of the "attractive" part. The consistent UI across vendors was something I really liked back when I was a Mac user.