You can propose a topic for a design meeting by opening an Design meeting proposal issue on the lang-team repository. We schedule meetings in our monthly planning meeting.
Every design meeting begins by reading a document and leaving comments. That document must be prepared 24 hours in advance of the design meeting and posted on the issue. If you propose an issue, you should be willing to prepare that document, or else indicate who will do the preparation.
Typically, design meetings are associated with active initiatives: you may wish to propose an initiative instead.