What are some of the classical references for instituting Hallel b'zman hazeh? And in a more discussion-oriented vein, what kind of miracle do you think it would take for the vast majority of klal Yisrael to agree on a new day for Hallel? Clearly Yom Ha'atzmaut and Yom Yerushalayim have not yet caught on in the broader charedi/yeshivish communities.
There is a famous Meiri (Rishon) which says that if an individual or a community experienced a salvation, they may recite Hallel every year on that day without a bracha. This is why some people say Hallel without a Bracha on Yom Hatzmaut.
מאירי פסחים קיז.
Classical texts (Rishonim) that I've seen talk about an obligation to say Hallel in recognition of great events benefiting the Jewish People.
As far as what it would take for agreement on a new day of Hallel, I personally think that Chanukkah is a great example. Like Yom Haatzmaut, it marked a return to Jewish sovereignty. But also similarly, there were many people, including some of the "Gedolim" of that generation who were very anti Chashmonaim. This is reflected in the Mishna's exclusion of Chanuka completely and the Gemara's relatively minor mention of it.
But eventually all Jews came to celebrate it. Also, in the Charedi community change is very very slow. I personally think that eventually the vast majority of Jews will come to celebrate Yom Haatzmaut, probably even with Hallel, but it will probably take another few decades.