What do you mean by unity? In principle I agree with one of the previous comments that our unity is in Christ. As far as I can see, in the NT there are a number of separate churches. They all honored Paul or some other founder, but there was no single leader in Rome or Jerusalem.
The more liberal churches went through this a few decades ago. We have sufficient doctrinal agreement that actual union would have been possible. Painful, as we'd have to agree upon a single way of organizing, but Canada and some other countries have done it. However most people didn't think it was worth doing. We already have intercommunion (although it's not always official), we regard ourselves as equally Christian, and local churches typically cooperate on things where it matters. I think that's sufficient.
If you look at the broader spectrum, I don't think organizational boundaries are that much of an issue anymore among Protestants. In the 19th Cent you denominations arguing with each other over who is the true Church. That no longer happens, except with Catholics and Orthodox, who for the moment I think are hopeless as far as Christian unity. Our main issue now is probably doctrinal disagreements, things like liberal, conservative, Pentecostal. Once you get outside the mainline denominations, you start seeing groups that won't have communion with others, which I consider a problem. But that's mostly caused by those disagreements. I just don't see how we can go much further as long as we have them. Nor do I see much hope of getting over them quickly.
I will say that if you watch things over centuries, I see progress. The Reformation created a problem. The Catholic Church had depended upon getting the State to kill dissenters. So once that stopped there was no way to deal with dissent. And the tradition inherited by Protestants from the Catholic Church said that there could be only one True Church, which had to have the same theology in detail, and not tolerate much in the way of variation. That model failed miserably in the new circumstances.
So once the State was no longer willing to enforce church unity, things kind of exploded. I believe it reached its worst in the 19th Cent. Since then, we've started seeing some improvements. The evangelical movement has unified a lot of the conservative side of Christianity. And they have some common statements such as the NAE statement and the Chicago statement on inerrancy. The mainline churches have largely buried the hatchet as well. My sense is that informally at least, we're going to see Protestants slowly collect into a half dozen or so groups that mostly recognize everyone else in that group as brothers, even if they're in different denominations. Getting those to unify is going to be harder.
One possibility is the evangelical movement. It has been slowly getting more liberal. The more liberal end of evangelicalism is now indistinguishable from the mainline doctrinally, but it retains its interest in spreading the Gospel. (So does the mainline, in theory. Our churches just mostly won't do what they need to to make it happen. They're not dead. There's lots of good Christian stuff going on. But there are specific things you need to do to grow, and for some reason most of the mainline won't do it.) I suspect that the mainline will continue shrinking, and will eventually just collapse into the liberal end of evangelicalism. That leaves us with a possibility that there will be a very large common core of Protestantism that's basically today's evangelicalism, with at least part of it more liberal. Whether it will be able to retain any coherence with such a large spectrum of belief is less clear though. I think there's a good chance of a split within evangelicalism. Probably driven by homosexuality. Yuck.
