Suggest
Ask for the thing that is missing.
Ideas are public, so other people can add to yours instead of opening a second copy of it. Which of the two below you pick decides what the idea is judged on.
A component that should exist
Open a component request. Describe the interaction, not the styling: a button styled differently from an existing button is not a new component here, and two ideas demonstrating the same behavior mean only the clearer one ships. The full standard is on the guidelines page.
Already built it? Skip the request and open the pull request through /submit.
A feature in the tooling
The CLI, the MCP server, this site, or the build tooling: open a feature request. Say what you were trying to do and where the current surface stopped you. That is the part that cannot be guessed later.
Before you open one
Search the open issues first, and add to the existing thread if the idea is already there. A reaction on someone else's issue is a real signal; a duplicate is not. Something broken rather than missing goes to /feedback.