Search the Add-ons page and see if any of the titles seem to match what you want.
Even if the idea is already implemented, many existing add-ons are no longer maintained and don’t work anymore in the latest versions of Anki. And even if they do work, you might make a better version.
Nothing is stopping you from stealing coding ideas from existing add-ons or simply copying them and modifying them, because all add-ons are open source. Ankiweb.net’s Terms and Conditions page states:
As add-ons extend the computer version, they must be licensed under the AGPL3 or a compatible license. If you do not explicitly state a license, it will be assumed to be AGPL3.
If there’s some add-on you’d wish to use and it doesn’t seem to exist yet, go ahead and write it and upload it. A few months ago I wrote a couple of very simple add-ons (1, 2) for my own use, using existing code from other add-ons as a guide. They’ve gotten only a handful of downloads but they haven’t been deleted.