Presumably, a Russian serf would mostly want to question his own ruler. Since Peter the Great is associated with the height of absolutism in Russia, we will look at what a serf might say to him.
We should assume that serfs would care most about the issues that really affected them. Therefore, they would not be all that interested in Peter's attempts to reduce the length of men's coats or to shave their beards. I also don't really see a serf challenging the Tsar on issues like absolutism because I don't think the serf would have cared if the Tsar had more or less power relative to local lords.
What I do think the serfs would have cared about was the taxes that Peter was levying on them. A serf might first have asked Peter why it was such a great idea to be more like the West. I think he might ask why it was so important to modernize the military and to build a new capital. Then, I would ask why the burden of these reforms had to fall on me and my people. I would ask why it was that I had to be the one to pay for these things that were never going to help me.
Since you are asking about serfs, I think you need to think about which things serfs would have cared about (issues that directly affected them) and which things (absolute monarchy or monarchy with less power) would not have mattered. I think that they would not have cared about great political issues and would instead have cared about things that made their lives harder.