I am a convert to Catholicism from evangelical Protestantism.
First, I do not believe the figure of 30,000 +, or 45,000 +, etc. My husband has done some research about this, and has come to the conclusion that the calculations are very strange. I would be careful about throwing these high numbers around. This is the kind of thing that will make thinking Protestants disregard anything else you say because they'll know that this "fact" isn't true and they'll distrust anything else you say.
Nevertheless, there are quite a few denominations of Protestantism.
My opinion is that unity will come about because the horror of sin will continue to increase in the world, especially in the United States, and this will force all Christians to come together and work to defeat the world, the flesh, and the devil.
We see this happening before our eyes right now in the U.S. President Obama's "mandate" forcing Catholic businesses to commit mortal sin is awful, and thankfully, Protestants are joining forces, at least with their words, to unite with Catholics and support them as they try to defeat this devilish mandate. Huckabee's comment bears contemplation: "We are all Catholics now." Wow, that's quite an amazing comment from a Christian who years ago, would have quite possibly considered Catholics "goddess cultists."
We've seen it happening in the last 40 years, as Catholics, Protestants, and the Orthodox come together to try to defeat Roe vs. Wade. Before RvW, it wasn't common at all for Catholics and Protestants to associate much with each other, other than in the secular realm. But when abortion actually became legal in the U.S., Catholics immediately went into action. Protestants were slower, because at that time, we honestly had no idea what was going on. I can remember sitting in Sunday school classes and hearing that sometimes, abortion was the best choice so that a child wouldn't suffer. You see, we were totally uninformed.
But as Catholics continued to hold the line on this battle, Protestants started waking up. The FIRST Protestant that I remember taking a firm and unapologetic stand AGAINST abortion was Dr. James Dobson of Focus on the Family. He was castigated at first by many Protestants who thought he was being "political," but as he continued to present the FACTS about abortion, evangelical Protestants began jumping on board and joining his fight to stop abortion.
One of the things that Dobson stated early on is that "Catholics puts Protestants to shame when it comes to fighting abortion." Very early on, back when many evangelical Protestant denominations taught that Catholicism was a "cult," Dr. Dobson stood alongside Catholics to fight abortion, and this was not lost on all the rest of us evangelical Protestants. Many of us started getting involved with pro-life work, and for the first time in our lives, spent a lot of time with Catholics--and we learned, to our surprise, that yes, they really are Christians! Wow!
So IMO, as horrible as the scourage of abortion is in the U.S., God has used it to begin to bring Protestants and Catholics together.
Another example is world hunger, which has its roots in human sin. The organization, Bread for the World, occasionally skirts liberalism, but often, the organization does a lot of good, too. There is a mix of Catholics and Protestants in this organization, too, and in fact, this was the first exposure I had to Catholics back in the 1980s--I did a lot of work with Bread for the World and met a lot of wonderful faith-filled Catholics.
What is frightening me right now in the United States is the growing power of the pro-gay-marriage movement in this country, which has paralyzed many Christians. We can't "protest" because in many cases, we will lose our jobs, our neighbors, our positions in various community organizations, and in some cases, our safety. At the moment, the Catholic Church is bravely holding the line on this issue, and although many Protestants agree, they do not have the unity of policy to be able to effectively mount a strong opposition to gay marriage policies, which are rapidly becoming the disgraceful law of the land. I think that this issue will cause many Protestants to draw even closer to the Catholic Church--they have to, because little fragmented "opposition" groups will fall, but a huge united force may have a chance of survival and success.
If you are interested in a wonderful book about the OPs topic, read Being the Body by Charles Colson. This book is about Christian unity, and is written by the co-founder of Evangelicals and Catholics together. Colson (RIP) has many pages of wonderful things to say about Catholicism in this book. This book, written by a leader of the evangelical Protestant world, is one of the main reasons I converted to Catholicism.