If present demographic trends continue, an Orthodox majority in the Knesset should be possible within the next few decades, perhaps 50 years or less. If the Orthodox Sephardim, Chassidim, Litvish, and Dati Leumi parties together have over half of the Knesset seats, they could technically form a coalition and govern.
But it wouldn’t be so simple as to simply change the laws in accordance with halacha, because each group has their own interpretations, and their own batei din for that matter (which already makes the existence of the Rabbanut kind of pointless, except as a way to assert dominance and get people jobs.)
Also, there will be a machlokes about to what degree it’s desireable or permitted to impose halacha on the chilonim before the arrival of Moshiach. Chabadniks, for example, would probably never agree to forcing the chilonim to do anything, or punishing them for transgressions, because that goes against everything their Rebbe ever said about kiruv. The dati leumi will probably be sensitive to the fears of the chilonim and keep them from leaving the country (to avoid an economic collapse, for one thing).
I think as a practical matter, democracy as we know it (as opposed to a theocracy run on non-democratic Torah laws) will have to continue until the Messianic era. It might even make more sense to have a separation of shul and state after an Orthodox majority emerges, because that way each community can rely to a large degree on their own autonomous batei din, and wouldn’t have to worry about the Rabbanut or the state getting in the way of their communities’ practices.
That doesn’t mean some things won’t change — I’m sure they will. Perhaps pork will be illegal. Perhaps some civil law and tort law principles will be changed to align with halacha. But I wouldn’t anticipate punishing Shabbos desecrators and heretics, and things of that nature.