American IPA is a mixed bag when it comes to adding things beyond hops- there are some additions that work pretty well, and some that don’t work very well at all.
Fruit seems to be the strongest categories of additions. Additions like cherry, orange peel, and lemon peel would pair well with the citrus flavor of an American IPA. It also provides a fairly good backbone for a berry addition, like raspberry, blackberry, strawberry or even cranberry. Pear also has the potential to blend well.
Tart ingredients in general seem to stand up pretty well with the hop bitterness, for instance hibiscus or lemon pepper. However, stay away from more floral additions like rose hips, elderflower or lavender- it’s likely that the hop aromas will either overpower or conflict with these additions. These would be more likely to work with an English IPA.
An American IPA is an okay base style for a spicy adjunct. Adjuncts for instance seeds of paradise, peppercorn, and rye would likely work. These are covered in the IPA substyles, and might move you into a separate category altogether.
BJCP Style: 21A