For me one main “must” in chili is to have a large diversity of chiles/pepper. Besides black pepper and regular chili powder (like McCormicks), I’ll add some powdered chipotle, a little cayenne, some regular and smoked sweet paprika, and dried New Mexico, ancho, and chile de arbol. (This is for a “red” chili - for a “white” chili like chicken chili I use white pepper and try to find as many varieties of green chiles as I can get.)
This is not to get it super spicy hot, necessarily, but I find it just adds a lot of depth of flavor. It might be just my imagination, but I’ve had people who barely know me come up at work potlucks etc. and mention similar. My sister visiting once with her family was very direct about it, because her husband’s chili is mono-dimensional habanero habanero habanero. But if I do want it hotter I’ll add some habanero or increase the amount of one of the hotter chiles (but not increase cayenne - if I get too much of that one it seems to stand out too sharply).
Other than the above, most of the time my chili is pretty freewheeling as to the rest of the components, but I do like to have a variety of beans, 3 or 4 types (normally canned because I can’t usually bestir myself to bother thinking about dinner early enough to soak).
About a month ago I followed a recipe from Serious Eats. Kenji wrote that he’d been asked to review someone’s “seriously best ever chili” (paraphrasing) recipe for an upcoming cookbook, and what he posted on SE was his improved version of that one. The main differences just from a basic style were a fair bit of chocolate, as many mentioned above, 4x more cinnamon than I normally use, and about 3x more nutmeg (I usually won’t use more than a gram or so in a gallon of chili).
I thought it was pretty good except that half the called for cinnamon would have been better. Not one of my kids or wife got seconds, which was a bad sign. They didn’t like the chocolate in it (but still next time I’m sneaking some in, just not as much) and especially thought the cinnamon was way overboard.
Now, all the above is for “sit down for a big bowl of chili” type chili. I also make a Cincinnati style chili for chili cheese dogs, but that’s more of a spiced ground beef sauce than anything. Except I can’t abide the idea of leaving all that fat in there so instead of boiling the ground beef straight off, I brown it first and defat.