My first thought was an alternate origin for the Sith and Jedi. Canon has the Jedi (or Jedi like groups) come first, but this doesn’t really make much sense given the mechanics of the force in Star Wars. If someone was self-teaching themselves how to use the force, and they ever used the force when angry or desperate, they'd find that using those emotions was both stronger and far easier than the emotion-less method, if they'd ever even stumbled on the emotionless method in the first place, which would be unlikely. From there, they've got a method which seems stronger and easier (ie. more "correct"), and unknown to them it's addictive. 99%+ of self-taught force users should end up dark side, it's a much more stable equilibrium for force use. In contrast, the emotionless method is a constant tightrope that if you ever fall off you end up darkside and don't come back from, it basically requires a formal order and training.
So in alternate origin, dark side proto-Sith warlords arise first. These warlords begin training other force sensitives in order to use as weapons against each other, which is successful enough that it becomes necessary, but they also keep getting too powerful and murdering the Warlord to take his place. Thus, the origins of Sith understanding of Master and Apprentice.
In order to avoid this, a Sith Warlord starts teaching his force-sensitive warriors to use the force wrong, teaching them an intentionally crippled version of the force, and killing any that use the full-strength force calling this the “dark side of the force." This turns out to be massively more successful than the Sith Warlord anticipated - his new breed of force sensitive warriors, the Ur-Jedi, were slightly weaker than the dark side acolytes used by other Sith, but their more pro-social nature meant that much larger armies of them could be fielded. Other Sith Warlords either rapidly followed suit, went into hiding, or were killed.