When IETF participants identify new work they would like to propose, that work may be in scope for an already-chartered IETF Working Group. If so, please contact the working group chairs to discuss it. Contact information for working group chairs is available at the IETF Datatracker.
If the proposed work is not in scope for an existing working group, the next step is to form a community of interested participants, and to begin planning to form a working group. A good first step is to request a non-working group mailing list, and then announce the mailing list when it's approved on appropriate existing mailing lists. The procedure to request a non-working group mailing list requires approval by an appropriate Area Director. This ensures that, if the discussions lead to forming a new working group, your proposal won't be a surprise when it arrives at the Internet Engineering Steering Group.
Often, discussions on a mailing list will result in a desire for requesting a Birds of a Feather session (BoF).