Sitcom, experimental or conventional is always about a bunch of characters locked together by profession or family or quasi-family.
So, brainstorm as many professions or types of family or quasi-family as you can, the odder the better, but don't critique yet. Just list. Think 'real but unsual' and go for quantity not quality. That way you let the cliches flush out.
You'll get twenty or thirty easily. Choose your favourite five.
Feel brilliant. (Yes of course, it clearly it's only the start. But not bad for five minutes... More tips on developing your ideas in later posts)