What to know when making my own campaign

so I'm going to start making my own campaign. Is there anything I should know? I've been dabbling in ally/villain sets so far, so I can make missions and agendas and such. Does there need to be a campaign gimmick? Or can I just start small and work my way out from there? Cause I want to make a campaign the size of core box/Hoth/Heart, but not sure where to start