I say mulling spices in one for sure: cinnamon, clove, nutmeg those kind of things
Then fruit, figuring something that would work well with apples. Perhaps pomegranate juice or blueberries?
And another suggestion I would go with would be abv boost. You could go with honey for a hybrid like cyser, or apple juice concentrate for a stronger flavor and a big boost.
Another cool idea someone on here came up with would be getting a caramel flavor in it. Using either caramel, caramel flavoring, or caramelize some sugar and add that in. At the very least it will give you a bit of a abv boost and darken up the cider a bit.