Other than Black Templars being loosely based on Medieval Knights Templars, and Bretonnians being loosely based on Medieval Knights, there isn't a tremendous amount between them. Dark Angels would be a closer match. They may not have the Vows but they are to all intents a Chapter based around Knights. Black Templars would be closer to the Empire and their Witch Hunters.
If Brets get redone then they could go towards the direction of Black Templars, but personally I think they would be better off going Holy through angelic imagery rather than Holy through "kill the witch" and a more darker look. There would still be room for a dark zealoty unit or two, but there isn't really a "divine purpose" (could argue Empire Sigmar) army which I think could save Brets from going the way of DoW.
I have to agree with Salt on this one. The Templars are religious zealots or extremists, taking an already severe religion to a whole new level. In Fantasy, the Empire is the Imperium- the main line of defense against Chaos, beset on all sides by inhuman foes, and maintaining grim superstition and heavy-handed fear tactics to prevent the spread of heresy within the populace. The Empire has the Witch Hunters, the Warrior Priests, and fanatical Doomsday Cults (flagellants).
In terms of fluff, the Brets have more in common with the Ultramarines, if anything. That's what I like about Fantasy over 40k- of the 15 armies in 40k, fully half of them represent "The Imperium" - while in Fantasy, each of the 15 armies represents an entirely autonomous kingdom, at war with each other on a regular basis, the only exception being Chaos Daemons, Beasts, and Warriors. Therefore, with only 7 real factions to choose from, I would say that really, there isn't a 40k counterpart to the Brets. If pressed, I would say Ultramarines, since they're portrayed as the least grim of the Imperium, but they're still just "Imperium+" and therefore seem more like Empire Reiksguard than a stand-alone army.
Dark Elves .. Dark Eldar
High Elves .. Eldar
Wood Elves .. Tau (loosely)
Orcs .. Orks
Warriors and Beastmen .. Chaos Marines
Daemons .. Daemons
Skaven .. Nids (loosely)
Vampires and Tomb Kings .. Necrons
The Empire .. Guard, Blood Angels, Dark Angels, Space Wolves, Sisters, Grey Knights, Space Marines
Dwarfs .. What's sad is that I even doubled up some of the Fantasy armies, there are just that many carbon-copy Imperium armies, worse yet because most of them are Space Marines, which is still worse because CSM is the same thing with different fluff, while Empire vs. Chaos Warrior is two polar opposite play styles. It would be like GW giving the Knights of the White Wolf an entire army book of their own, or having a different book for each Chaos God.
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