Been thinking and have some (honestly not great) guesses about the legendary traits for each Legion. Arranged in no particular order
Loyalist
Dark Angels - Lionel's killing intent. Disregard honor, learn to put entire worlds to the torch.
Ultramarines - Adaptability. They will never fall for the same trick twice and will create countermeasure for any scenario they experienced.
White Scars - Something about wisdom? The khan had some of the best reads on literally everyone and consistently made good decisions.
Salamanders - Master Craftsmen!
Iron Hands - Ferrus's drive to constant improvement, affinity for armored warfare or cult mechanicus relations.
Blood Angels - Being the most perfect and best Space Marines to ever live.
Raven Guard - Guerilla warfare wirh a side of 'the chapter never quite goes extinct'
imperial Fists - Dorne's rage, Fortification, and maybe space combat?
Space Wolves - Champion Hunting or affecting morale seem the most obvious.
Traitors
Word Bearers - Absolute belief in the Imperium and or eternal love of the eclesiarchy.
World Eaters - Empathy, teamwork, or just being the absolute killiest thing alive.
Death Guard - Incredible durability and general tirelessness, absolute hatred of the xenos and or psykers.
Thousand Sons - Psykers, potentially also a 'know your enemy' type bonus for fighting known foes.
Night Lords - Prophecy and terror tactics are the only options I see.
Sons of Horus - Befriending ability, tip of the spear actions, and or inflicting terror (again)
Iron Warriors - Spiiiiiite
Emepror's Children - Incredible specialization and a higher ceiling for average skill
Alpha Legion - impersonating anyone, absolute bullshit improv ability.