Can be any format.
For Commander decks, I have at least 5 Dragon tribal decks already and am planning to try to build a dragon tribal deck out of every possible multi-color combination plus mono-red eventually. But Slivers and Eldrazi are up there too as two of my favorite tribes to build around.
Cephalids! Don’t even know where it came from but at some point I got the idea to make a deck around Aboshan so I ended up going full cephalid tribal and while it’s not very good I love it anyway.