Nice seeing this going around again!
My standard cake is box mix + milk for water + melted butter for oil + dash vanilla extract + frosting from scratch. This really seems to hit the right spot for people of “mmm, homemade” but also “exactly like Mom used to make.” (Do that for a yellow cake with chocolate buttercream frosting, add candles, and serve to a college student, for the maximum “this is exactly what I didn’t want to admit I wanted” potential.)
Seconding the addition of coffee to chocolate cake; a tablespoon of instant coffee powder in a dark chocolate cake makes it taste chocolatey-er without actually adding a perceptible coffee flavor (I don’t like coffee flavor, personally, and I still do this).
Another good option is a box lemon cake mix plus maybe 3 lemons. Zest the lemons, set the zest aside, then juice them and use that in place of the water; then use the zest to flavor the frosting. Adds a nice fresh kick.
Chocolate chips can be dumped straight into chocolate cake mix without fussing with anything to compensate. Sprinkles can go into white cake mix to make your own “confetti cake” with any specific color combo you like. Any kind of dried fruit can be chopped to raisin-size, soaked in hot water (or, better yet, hot juice with a couple of citrus peels added) for an hour, drained, and then added to batter.
Replacing part (up to maybe 1/3) of the water with yogurt (and then the rest with milk as usual) will give you a denser cake; make sure to check if it’s cooked through, and bake a little longer if necessary.
Swirling things through batter for that fancy marbled look is easy. Consider melting chocolate chips with butter, or mixing brown sugar with cinnamon and a little melted butter, or making up two different cake mixes and swirling those together.
I swear by the Cake Mix Doctor’s two cookbooks (one’s general, one’s specifically for chocolate cakes). I think every birthday cake I had as a child was out of those.