One of the features I use most is, "Hey Alexa, add (insert task) to my to-do list." I use the AnyDo app to integrate with this feature.
Something I would like to see Echo capable of doing is adding tasks to a specific to-do list on AnyDo. For example, "Alexa, add (pay cable bill) to my (personal) to-do list." Currently Alexa can only add tasks to the Alexa to-do list.
Also, when I ask "Alexa, what is on my to-do list?" I would like it to tell me what is on all my different to-do lists, not just the Alexa list. For example, "You have two tasks on your (personal) to-do list, and two tasks on your (work) to-do list. They are..."