What you're really asking is whether God still inspires people. I think most Christians would accept that He still does. So, the principle usually pointed to is that expressed in 1 John 4.
Put simply, any new "revelation" can't contradict the Bible. So, if it agrees with the Bible, it would be accepted. If it disagrees, it would be rejected.
At the same time, I don't see why anything would ever need to be added to the Bible. In terms of what needs to be said, Jesus summed it up in John 19:30, "It is finished." The saving work of Christ is complete, so there is no need to enhance the theology, philosophy, or whatever else people might think to write about.
In that vein, I would expect that any inspiration that happens today would be specific to a situation.
FYI, western history sometimes claims that Mohammed's original goal was to convince the Jews that he was the successor to (or perfector of) Jesus' message. It was only when that didn't go well that he claimed a separate religion. Of course Muslims reject that idea, and no one today really thinks of Islam that way, but there are verses in the Koran that could be used to support that idea.
