8th Edition 40k is the most lore-friendly version of a game rich in background, themes and its own unique flavour. Many armies have rules, stratagems and abilities to bring them to life in the game as fans imagine them, but many Black Templars players feel we've lost too many rules that gave us our individual style of play and a strong link to our rich narrative. Despite 9 pages in the current Space Marines codex, more characters and models than most other chapters, and our own stratagem, relic and tactics, something is missing. Other zealous armies in 8th edition have rules that make them different, instantly recognisable as fanatics. Adeptus Ministorum/Sisters of Battle have Acts of Faith and automatically Deny The Witch, Adeptus Mechanicus have Canticles of the Omnissiah, while Black Templars are becoming ever harder to tell apart from other chapters. 

Vows, oaths of fire and blood, wouldn't break the army. They could be tailored to fit, play-tested easily and refined then beta-tested. As new rules they could be added to Chapter Approved or a pdf from Warhammer Community. It's a small change that would make a huge difference to the way a much-loved but out of the limelight army feels to play in a game that succeeds in so many ways already.