With Reaper of Souls confirmed to not only be taking place in Westmarch - the location of the Templar's stronghold - but also said to involve side-quests for each of the Followers, the Templars should have a role to play. Especially with their lore that we uncover by talking to Kormac as we play through the game.
If you've gone through all of Kormac's dialogues, you can see what role they might play in Reaper of Souls. They have tomes of prophecies, they were possibly created to stop an angel invasion, and somehow they acquired plans for angelic weaponry to fight them with.
The questions then are: Who established them? Since they have plans to make angelic weaponry, does that mean it was an angel? Was it the Enchantress' Prophet? Was the threat they were designed to thwart something in the past, or was it foreseen that Malthael would be coming as the Angel of Death? (and it has been confirmed he wants to destroy all humans)
Unfortunately, we might just fight them since our character instantly distrusted them in our conversations with Kormac, and Kormac wanted to go challenge them and get justice for what they've done. If we don't, they could be a powerful ally against Malthael.
We don't know too much about the Templars, but what we do know is pretty interesting.
The Templars are an ascetic order based out of Westmarch. They are holy warriors, fighting for justice and eschewing the dangers that lead to cultism and demonic possession by espousing self-denial. They do not merely pay lip service to protecting the realm - they are battle-trained and experienced to a man.
The great bastion of the order stands astride Westmarch, It has been besieged many times and even fallen, but never for long, as the Templars will 'rebuild a thousand times if they must, but will never give in.' There is something called 'Vault of the Order' there, which could be the name of the stronghold or an actual vault, as the Templars also collect chalices, vials and sacred texts, which 'have a tendency to inspire a deep reverence in those who view or hold them.'
The order is headed by an individual called the Grand Maester. There is also a group called the inquisitors, which recruit new members. The Templars' recruits are supposedly criminals who are stripped of their sins and the memory of their past life by the inquisitors. Apparently they have been abducting innocent citizens and subjecting them to horrific torture, then purging their memories and molding them into zealous adherents. (This may be an 'ends justify the means' thing since we saw for ourselves how the 'Templars were established to fight angels' secret went over with the Nephalem and Kormac. If your order was tasked with such a deed, it would probably be hard to find recruits. :cute:)
In the Book of Tyrael, Tyrael says he can't confirm any of these darker tales with facts, but the mere existence of them makes him fearful of the group's true intentions.
The Templars are trained to use the Light, and are 'turned into a living weapon, a light against the darkness.' They are said to practice throwing spears, and possess catapults.
A key part of the Templar lore are their sacred tomes, said to contain prophecies. Unfortunately, I believe when/if Kormac talks of these it's done 'in the field' which makes it a little difficult to find where and when he mentions them. I believe the prophecies are supposed to be akin to the End of Days prophecy though.
These tomes were stolen by the Archbishop Lazarus at some point. How might Lazarus have accomplished this? As King Leoric succumbed to demonic oppression and went mad, Lazarus eventually convinced him to wage war with Westmarch, perhaps distracting the Templars and allowing Lazarus to make off with their tomes.
Why did Lazarus steal these tomes? We know Lazarus was corrupted and working as an agent of the Prime Evils, so he may have stolen them to deprive a threat of any forewarning towards what Diablo and his brothers had planned, or to give the Evils whatever information the prophecies contained.
(These sacred tomes may be the source for the excerpt of the Prophecies of the Final Day seen in D2:LoD's manual. Ord Rekar, an Elder of Harrogath, is familiar with these prophecies, and the Barbarian is the only character (currently) that knows what the Templar are, as he/she tells Kormac that he/she knows Templars can be trusted when they first meet. Of course the Barbarian could know of the Templars' good character else wise, but it would be a cool reference if they were the source of that LoD prophecy. ;))
These Templar prophecies likely describe the events of Diablo III/Reaper of Souls and beyond. Like the Druids, it appears they have been training for a final, prophecized battle. The Templar initiation 'blinds us to our sinful pasts so that we might clearly see the future and prepare for the final battle."
When your character hears about the initiation process and is appalled, Kormac replies, "Don't you understand? The Templar order is trying to protect humanity from extinction."
I don't believe Kormac finds the tomes during the events of Diablo III, but he does find the torn page that Jondar had hidden, 'as if proof of a crime.' The page does not contain the prophecies that Kormac expected, but instead had 'a set of instructions for the creation of magical weapons' and an angelic rune, the same rune Kormac had seen on Tyrael's bracers.
You and Kormac bring the page to Tyrael, who instantly recognizes it as plans for 'angelic weapons that will fit human hands.' Tyrael deduces that his brethren (the angels) sought to destroy Sanctuary, and that the Templar order was created to stop them. Despite the fact that Tyrael, the Archangel of Justice himself, admits that the angels did indeed wish to destroy the world, the player character and Kormac both think that the Templar order must be 'bad' because of this, and Kormac vows to go plunge his spear into the ground before the Vault of the Order and "meet their champions until they answer for their sins and their lies. I will have justice."
We encounter two Templars in the base game:
Jondar - The 'Necromancer Betrayer,' Jondar was once a man of faith, devoted to the Templar order. We don't have too much background on this character, but like Kormac and other Templar recruits, Jondar suffered the same initiation ritual and had his past memories removed. Presumably during his search for the sacred tomes of the Templars, Jondar came across 'key words' in a dusty libram. Reading these words undoes the inquisitor's brainwashing, and afterwards Jondar began to mistrust the order.
Jondar likely discovered one of the order's sacred tomes, as he had a ripped-out page from one and had studied it. It is unknown if Jondar knew exactly what he had found (the page contained plans for angelic weaponry fit for human hands), but he knew it was important enough to hide within the binding on his journal.
Jondar is possibly mind controlled by the Coven (likely via the same ritual we interrupt them performing on Kormac), and upon his defeat he asks Kormac for forgiveness, stating that his vision had been clouded by the Coven's evil magic. Kormac immediately strikes him down, for betrayal can never be forgiven.
Later, after realizing the truth about the order that Jondar had discovered, Kormac feels some regret for slaying his friend. He knew Jondar was troubled, but said nothing, and dismissed Jondar's doubts as weakness. The Barbarian counters by saying that Jondar's blood is on the order's hands, not Kormac's, and Kormac agrees and says the order shall pay for Jondar's downfall. The Wizard says while he/she personally thinks everyone can be redeemed, he/she thinks Kormac made the right choice, which comforts Kormac, who even admits that Jondar was able to tell stories that made him laugh, if you can believe it. The Witch Doctor says that Jondar's spirit is now free and likely doesn't blame Kormac, which also comforts Kormac. The Demon Hunter and Monk fail to console Kormac, their replies merely make him wonder if he could have said something, done something, to change Jondar's mind.
It would be interesting to know when Jondar became a Necromancer. Was he one in his life before being brainwashed into the Templars? If he was, did he still have necromantic powers while a Templar? Was it something he became after the Coven got ahold of him? Or did he realize the dark truth to the Templars, that they were created to stop the angels from destroying the world, and this revelation brought him into the Cult of Rathma aka Necromancers, which are supposed to uphold the Balance between Order and Chaos.
Kormac - A paragon of the order and considered one of their best warriors, Kormac was possibly tasked with recovering the order's sacred tomes, destroying the Skeleton King, and making his fellow Templar and friend Jondar pay for forsaking his vows and joining the Dark Coven. He joins the Nephalem on our quest, and along the way he discovers some secrets about the Templars and himself.
Kormac is also quite taken with Eirena the Enchantress, but tries to adhere to his order's ideals against love, since it "is the surest way to a tragic end." While not selected as your follower in Bastion's Keep, one of his lines asks the Nephalem to do him a favor and let Kormac come with us for a while, because, "These widows keep pestering me."
The player character finds the Templars' memory wiping initiation highly suspicious, and plants a seed of doubt in Kormac's mind. Kormac decides to try and recover his memories, in part to exonerate his order. He eventually finds the method in the journal he took from Jondar's corpse. He discovers he was once a promising soldier of Westmarch, until the Templar recruiters lied to him and piled false sins upon him.
Despite learning of his impressment into the Templars, Kormac still thought Jondar was a fool, because he believes there is no way the Grand Maester knew what the inquisitors were doing. When he finds the hidden page and learns what it represents, Kormac says his faith is stronger than this betrayal. After defeating Diablo, he vows to go back to Westmarch and make the order answer for their sins and their lies.