Could Jesus return right now?

I just read a chapter in Grudem's Systematic Theology on The Return of Christ. It mainly looked as the aspect's of Christ's return that all Bible-believing Christians believe. These are things such as Jesus will return as a person, visibly, bodily; as believers we are to eagerly await his return, we do not know when he will return, etc.

An interesting endpoint of this chapter focused on the question of whether Christ could return at any time. To be more specific, could he return right now.... or now.. or now?

The thing that makes this question interesting is that there are some things written in the Bible that are suppose to happen before Christ comes to earth for a second time. For instance, the gospel is supposed to be preached to all nations first. Or there is supposed to be a great tribulation. Or has this happened yet? These are some of the things that are supposed to precede Christ's return. So can we really say that Christ could return right now?

Based on the first reason, that the gospel hasn't been preached to all nations, I was convinced that Jesus not only would not, but could not return until  that happened.

Grudem suggests another way to look at it. He suggests that for each sign that is supposed to occur before Christ returns, we can say that it is unlikely, but possible that the sign has been fulfilled already. For me that biggest hang up with this view is the fact that the gospel hasn't been preached to all nations (nations = people groups). But then what does this verse mean? Or this? From these verses I could say that it is unlikely, but possible that the sign has already been fulfilled.

So maybe it is fair to say that it is unlikely, but possible that Christ could return this hour. What do you think?