In the time of the Stormcloak Rebellion and the Dragon Crisis, the Thu'um was more or less a lost art, practiced only by the Greybeards and exceptions like Ulfric Stormcloak. However, Ulfric's use of the Thu'um in his duel with High King Torygg and the Dragonborn's wide us of the Thu'um to bring down Alduin and many other Dragons, could potentially cause a resurgence in the Thu'um popularity. This could mean that we could be seeing the Thu'um in some small ways in future games.

Tiber Septim founded the Imperial College of the Voice in Markarth Side, though no trace of the original College remains. Markarth and the Reach have and will seemingly continue to experience great change regarding the Reachmen, the Nords and Imperial authority in the region. As of the events of TES VI, we could be hearing rumors of a new College of the Voice in Markarth, where new warrior-Tongues are being trained in the Way of the Voice. Perhaps even a Tongue follower from there.

Ulfric Stormcloak's use of the Thu'um is a profane violation of the "Speak Only In True Need" creed of the Greybeards. Especially when he used it in an act of regicide.

I, myself would like to see this happen. Unlikely since in-game, there doesn't really seem to be anyone willing/able/or even have the ability to use the Thu'um (good luck finding someone whose even self-taught) to teach it to anyone, so phase one is down the tube. I'd like to think some obscure clans/people maintain the Thu'um in remote regions, but they tend to do their own thing with it as opposed to militarizing it.

I doubt we'll be seeing the Thu'um in such a manner. The Dragonborn is unable to experience the difficulty and challenge that regular mortals face when trying to learn it, so I doubt they could do so even if they decided "screw the Greybeards." As for Ulfric, even if is alive in the next game, he'd be too busy to teach others to use it, and I believe he respects the Greybeards enough to not cheapen it to that degree.

Err... what? Objective evidence of his actions flies in the face of that claim. He used it to attack the Forsworn. He used it straight out of the gate when committing regicide under the guise of an "honor duel". He has no respect for the Greybeards or their tenets, especially that of "Speak Only In True Need". Actions speak louder than words, and while his words pay lip service to the Greybeards, his actions show that he's willing to abuse the Thu'um to get his way. Jurgen Windcaller would probably give him an earful of reproach when they meet in Sovngarde.

I think it would greatly depend on what Skyrim would look like post-game. Ulfric could potentially pass on his knowledge to some of his high-ranking officers, or even attempt to create some sort of Tongue regiment, to better combat the the Empire and those that would oppose him, perhaps even creating a Thu'um wielding upper class of Nord warriors. The Dragonborn's actions in Skyrim could also inspire the Nords to pursue knowledge of the Way of the Voice, which the Greybeards might offer, should something like their leader Paarthurnax dying happen and endanger the future of their order. The Dragonborn God defeating the World-Eater could also trigger a resurgence of the Old Nordic Pantheon, I surely hope an independent Skyrim wouldn't continue to revere the Imperial Pantheon for long before switching back to the old one.

Perhaps a follower, some warrior-monk Tongue from Skyrim in either High Rock or Hammerfell.