I'm challenged by photographing strangers. It's really hard for me to walk up to someone and ask to take their picture. I don't even know what I'm afraid of, as I'm not normally a very shy person.
That's a boundary I'm looking to push through.
To get really out of my comfort zone, perhaps go to Sturgis for one of the biker meetings there, and do a documentary. On film. That would be challenging to me. The hardest thing I know is to capture fleeting moments. Landscape, aerial, architecture, etc takes mostly time, dedication, and enough money to pull off. A fleeting moment never returns, so it becomes an exercise of reacting quickly and making very fast decisions. That, to me, is a lot more difficult, and to do that with a stranger even more so.