We have a steering committee. The committee's main responsibility is to set the agenda for each meeting.
Stuff gets onto the parking lot one of two ways.
The intention is to identify new business, determine the scope of the issue, whether to refer it to a committee, etc., and get it onto the parking lot for a future meeting. Proposals do not ordinarily come from an individual, but usually are sponsored by a committee with a recommendation. Some things do not make it to the agenda, and either become non-issues or are resolved some other way by the time they surface in the priority list or there is finally enough time to deal with them. We try to stay out of the two not important zones. Of course if something really important is being overlooked, it will resurface or be championed by someone to get the proper attention.