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.
Sure! I’m still in the process of upgrading it so this isn’t the most optimized list in the world, but it’s still run really well so far. Naya is one of my favorite color combos because I just love creature decks.
I also have a Historic Brawl list for arena that’s a bit more optimized, here, for some more ideas for cards, disregarding budget.