If using frozen corn and you don’t have time to thaw it, there are two good options. First, you can stir it into the hot rice. Or if you cooked the rice earlier, add the corn on top of the black beans when rinsing in the colander. The kernels will quickly thaw when the water is run over top. Drain well before adding to the rice.
A chopped avocado may also be stirred into the rice salad. If you expect leftovers, however, you may wish to use it as a topping, as the avocado will brown over time.
If expecting leftovers, you may wish to make 1½ to 2 times the salsa-mayo-cumin mixture. That way, you can hold some aside to add to the leftovers if they seem dry, as rice absorbs a lot of moisture over time. Alternatively, you can simply stir in an extra spoonful of salsa and mayo (plus a pinch of cumin) to leftovers if needed. If you end up with extra dressing, it’s delicious as a condiment for sandwiches, burgers, roasted or baked potatoes, even simply baked chicken.