I really like building themed decks too, though I aim to be pretty competitive at standard events with lots of layers of strategy and combo's etc. I've found that instead of tribal decks or straight up "every creature needs to be a zombie" I like building more of a... short story... if you will. SOI also gives a great outlet for the H.P. Lovecraft kind of stories I love.
-shameless plug-
This is the deck I've been working on lately -/shameless plug-
I have a soft spot for all of the cards there, but the goal was to tell a story that from the 'beginning' (1-2 drop creatures) there were good intentions, to defend and protect, to become sages to a different race to help them
thrive (before you have 6 land, which I would consider the 'end'), to become leaders of a military to fight against the
darkness. But slowly these intentions turn into (3-4 drop all the way to 6 drop) unending corpses, leaders of the military becoming leaders of demonic cults, giants filled with zombified victims, traitors of their kin, demons consumed with madness, angels of
fire purging those they were sworn to fight side by side with, swathes of creatures being sacrificed to the profaned prince.
I think this way of thinking about a theme much more interesting, and a lot less 1 dimensional than tribal decks. Because it's multi-dimensional it also tends to perform better, which I'm loving. That's my 2 cents anyways, this kind of story put behind a deck really keeps me interested and enjoying playing with it for a long long time, and I agree, it's very difficult for me to put in a card that doesn't quite fit in thematically