Generally speaking Split evolutionary lines have split off from the base or second stage, with the two stages being identical to each other. There has been some fan variations on this, for example, the completely different lines Wurmple goes through has been used for the basis for completely split off two separate lines. Here's some other ideas:
1."Quick Evolutions": Alternate evolutions for single stages of three stage Pokémon, these would be more easily accessible than standard three stages. Their final stats will be determined considering how the base Pokémon would be if it was the first stage of a two stage Pokémon.
For example, Mareep regularly evolves into Ampharos. But, with a new item, let's say Quick Trigger, it will evolve into species that's around 400-480 BST range, with one stat around 100 range. This new evolution would more closely reflect Mareep's stats, with slightly more speed than Ampharos, higher physical defense, and a completely new additional type. This new evolution would work better in-game, but would not be significant in competitive metagame without proper investments.
2."Slow Evolutions": Same idea as above, but instead of species getting a quick alternative evolution, single stage Pokémon would instead get three stage Pokémon evolutions.
For example, Voltorb can directly evolve into Electrode, or it can wait a while with "Slow Trigger" and become a Great Ball Pokémon with Steel subtyping, like its partner in-crime Magneton, and gain a third evolution who gets Levitate and is based off an Ultra Ball.
3."Convergent Evolutions": A completely different than the above, instead of a species getting an alternate evolution, an already evolved would instead get another pre-evolved species group that would evolve into it. This would be primarily used on split evolutions to give them conventional pre-evolutions, and rare species who differ significantly from their original form to obtain them in different regions.
For example, Bellossom would have another middle stage and a starting stage. These species would be fit for tropical environments as opposed to the nocturnal Oddish. This new middle stage would evolve into Bellossom with regular Leaf Stone, while evolving into Vileplume with Moon Stone.
In the latter case, Scizor would get another pre-evolution that is adapted to desert environment. This new pre-evolution evolve into Scizor through natural means, let's say around Level 35, and make Scizor available in the wild. However, this species would lose on Scyther's pre-evolution moves.