You get to ask only 1 question to the you of the future.
What would you ask?
"What do you wish most I had done differently in this period of my life and why?"
I often imagine communicating with my future self, usually when I am frustrated or anxious, or disappointed about a relationship, as a way to gain perspective. I do this in both directions, thinking back to times in the past when I've imagined my future self and deciding what I would say now to me then.
With regard to disappointments, I often realize looking back that I'm glad I didn't get what I wished for.
I have some pretty good ideas of the advice I would give younger me, so I think I would ask him, "What advice do you have for me". He's got a better idea than I do.