Milton wrote about the most critical issues facing human beings, believed in freedom of thought, and is considered to be one of the most important writers in the English language.
One reason why it would be interesting to meet Milton is because he writes about the basic elements of human existence. In his epic poem Paradise Lost, Milton argued that his purpose in composition was to "justify the ways of God to men." Anyone who sets out with that purpose in their writing has to be a little bit interesting. Milton tackled "big issues" and in talking about how God relates to humanity, he takes on one of the biggest ones out there. In meeting him, it might be interesting to get his take on human beings, the notion of sin, and whether or not he thinks that human beings can be redeemed. Asking him to address these issues in the modern day setting would be really interesting.
Milton was a passionate defender of freedom of expression. In Areopagitica, Milton talks about how a society cannot function without freedom of thought. Censorship cannot be tolerated. Government's reach cannot extend into limiting the thought of its people. It might be interesting to see how he would respond to issues of control today. Are we artistically and intellectually free? Would Milton say that we have advanced in our ability to take ownership of what we think and say? Are we respected in doing so? I think that it would be interesting to meet him and get his insight on how things have or have not changed since he published Areopagitica.
Finally, Milton's place is a giant in English literature. It would be interesting to get him to speak about how he perceives himself. If you had a chance to meet someone like Milton, Dante, or Shakespeare, it would be interesting to simply get them to talk about how they have been enshrined in this literary hall of fame. Did Milton expect to be counted among the greats? Does he even see himself there? Does he see himself as better than someone like Shakespeare? Since titles of "greatest writer" are generally conferred upon one after one's death, it might be interesting to meet with Milton to see how he views himself in relation to how others see him.
I shudder to give a final reason because it is not on the same level as the others identified. Milton suffered blindness toward the end of his life. It might be interesting to talk to him about what a giant of intellectual thought experienced in losing his sight. He is so much more interesting than simply a physical condition. However, if one more reason is needed as to why he might be interesting to meet, this one could suffice.