Iron Bull was one of Dragon Age: Inquisition's most well received characters by fans, and it would seem wasteful to overlook him in Dragon Age 4, which is currently in development. Given the context of the political conflicts engulfing Thedas at the end of Dragon Age: Inquisition, particularly the new Qunari invasions, it would seem necessary that Iron Bull and his mercenary band naturally have a role within this paradigm.
While much of Dragon Age 4's themes appear to be focused on Solas and other potential antagonists, it is clear that the Qun are exploiting matters and focusing on invading Thedas. Given that player choices in Dragon Age: Inquisition were designed in such a way that Iron Bull only evades death if he remains loyal to the Inquisition, it seems plausible that Bioware could possibly have a particular plan or story arc for Iron Bull in the next game.
To speculate the whereabouts of Iron Bull after Dragon Age: Inquisition and Trespasser DLC, a basic understanding of the Qunari culture and society helps. First, Qunari society follows a strict religion called the Qun, which governs their island-homelands through a theocratic system which demands much obedience to it. If player choices ensure that Iron Bull lives on, he is ultimately cast out of the Qun in Dragon Age: Inquisition. This can clarify matters and predict where the storyline of Iron Bull could head towards, while choosing to keep him loyal to the Qun has bigger consequences.
Where do the Qunari fit into Thedas' landscape?
The Qunari race are complex among Dragon Age's races, given that they are both a religious ruling structure and a race. Specifically, the Qunari are its people, and the Qun is the religion. Those that belong to the Qun see little distinction between the race and its culture, and the religious theocracy that governs the race itself. But not all among this race are part of the Qun. Some Qunari leave their island-homelands, often to become mercenaries in other parts of Thedas and are considered exiles and no longer considered part of the Qun, and neither are they Qunari at that point. While, biologically, it could be considered the same race, they do not see it that way.
Before the events of Dragon Age 2, it was incredibly rare for civilians to spot a Qunari in the mainlands of Thedas. But in this new paradigm, heading into Dragon Age 4, it appears that Thedas will now have a mix of both Qunari who are still loyal and part of the Qun, and exiled Qunari known as the "true grey ones", or Tal-Vashoth, who have resided on Thedas' mainlands far longer than them. By the end of Dragon Age: Inquisition and its expansions, Qunari forces invade Thedas' mainlands in mass, posing a grave threat.
What is less clear is how the Tal-Vashoth and Qunari will deal with each other in this potential conflict, and where their loyalties will gravitate towards. It would seem that some Qunari and Tal-Vashoth alike may have issues and conflicts among their own race, despite allegiances and varying beliefs over whether they are actually the same race. It's always possible fans could see the Qun work towards reconciling Tal-Vashoth mercenaries back into their society and ranks as a means for gaining more power to their forces, while also reducing the forces of their enemies.
Some Tal-Vashoth may find such possibilities enticing as well, given that much of society may likely begin to distrust them and view them as savages, given the invasions by the Qunari. This was already demonstrated to a degree in Dragon Age: Inquisition if players made the tough choice in Dragon Age that turns Iron Bull on the Inquisitor. This leads many people of Thedas to distrust Tal-Vashoth mercenaries, comprising long held relations with them.
Where does Iron Bull fit into all of this?
Iron Bull is a nuanced character and Qunari at the start of Dragon Age: Inquisition, given his lifelong obedience and belief in the Qun, and serving it, while also having been forcefully exiled from it later on. In other words, he does not evenly fit the mold of a traditional Qunari or an exiled one, since the latter likely reject or do not closely adhere to the belief, ideology, and lifestyle of the Qun's governing system and overall beliefs. Iron Bull falls somewhere in the middle, even at the start of the game, and ultimately remains loyal to Dragon Age's main character if players make the right choices to keep him alive.
In the events of Dragon Age: Inquisition, Iron Bull is working undercover for the Qun as an exiled Qunari mercenary. In his undercover role, he led a band of exiled mercenaries known as Bull's Chargers, and the Qun utilized it in order to gain intelligence on Orlesian nobles, who often hire these mercenaries and work closely with them. After many years, Iron Bull develops a complex identity-crisis from working undercover, which undermines his absolute loyalty or belief in the Qun and it complicates matters greatly. In a way, he is one who respects and believes in the Qun's mission and overall philosophy, but does not wish to practice or live according to its lifestyle.
Determining the true threat of Corypheus and the Venatori to Thedas in Dragon Age: Inquisition, the Qun are willing to create an alliance with the Inquisition. As Bull's Chargers enter battle against the Venatori on the Storm Coast, players are forced to choose between sacrificing the lives of the Chargers in order to win the battle, or save their lives by retreating. If Iron Bull saves his comrades, then he is officially cast out of the Qun and made an enemy to it.
Regardless of the player choices in Dragon Age: Inquisition, Iron Bull proves himself to be a compassionate and empathetic character who was cares deeply about his comrades, and even in the event in which he feels obligated to sacrifice them, it is a deeply and emotionally troubling event for him.
In the events of the Trespasser DLC that comes later, Iron Bull is killed if players chose to have him turn on the Chargers in the base game. This means that if he has a role in Dragon Age 4, it will be one in which he is likely exiled from the Qun and loyal to whatever remains of the Inquisition.
Qunari Invasions Conclude Dragon Age: Inquisition
As Qunari forces begin to invade the mainlands of Dragon Age's Thedas after the main threats in Dragon Age: Inquisition are disposed of by the end of Trespasser, we could see a more heated conflict between the Qun's forces and Iron Bull himself. Perhaps we may even see a more nuanced version of Iron Bull whose lifelong belief in the Qun, yet his years of service as effectively Tal-Vashoth leads to a further identity-crisis which could test the loyalties of his character once again, especially if the Qun offer him reconciliation.
In the big picture, it is a very strong possibility that Iron Bull will be between a rock and a hard place, given all the events transpiring in Thedas. But the ultimate tribulation in this conflicted landscape could be one of which is a test of allegiances and trust within the ranks of Iron Bull and his Chargers, and the Tal-Vashoth as a whole. It will also test other races who belong to the Qun's religion and overall system, but may not be a Qunari, such as Elves or Dwarves. New invasions by the Qun could certainly create social rifts within Thedas' society as a whole, especially among believers in the Qun who may actually be alarmed at the Qunari invasions.
Since Iron Bull is killed in one of the story arcs in Dragon Age: Inquisition, it is doubtful that he could be a main character in Dragon Age 4, as there would need to be a version of the main storyline that can proceed forward with or without his inclusion. Though there are certainly ways around this, the easiest one being to replace his character with a different Tal-Vashoth character if he died in the events of Dragon Age: Inquisition.
Returning to a role as a strong Dragon Age companion or some other role, such as an NPC, would certainly delight many of Dragon Age's avid fans. While much of Dragon Age 4 is still left for speculation, the conclusion of Dragon Age: Inquisition appears to create very ripe and welcoming conditions for a return of Iron Bull and his mercenary band, if Bioware so desires it.
Dragon Age 4 is currently in development.