Some non legendary examples I can think of are
If you have a beldum or metang in your party, more beldums and metangs will spawn because of the logic of how they evolve in the Pokedex. When two beldums get together they evolve into metang and when two metang get together they evolve into Metagross.
This could also be applied to Magnemite spawning more Magnemite or Diancie spawning more carbink, if I spent some more time going through every Pokedex entry to figure this out I could give more examples.
But then you could take it a step further to remove some of the waiting around from legendary spawning.
If you have a Latios on your team then Latias can spawn in any biome because they are usually always around one another.
If you have a Kyogre then both Rayquaza and Groudon have a chance of spawning in any biome, and if you have two of them then the third is significantly more likely to spawn.
I could go on but I hope you get the idea.
I feel this would remove more of the luck based mechanics that a lot of people seem to dislike without having, especially around legendary pokemon, without having to add in structures and items for each of them to be made completely skill based.
Now onto why this shouldn't be a feature
On servers that have it could cause more lag with having to check player teams every once in a while when a legendary tick happens or when Pixelmon need to be spawned to fill the mob cap.
This could be an impossible feature to add due to the nature of the spawning in its current form.
If anyone has any more examples off the top of their head as to what pokemon it would make sense to effect and what reasons it shouldn't be added please feel free to respond as to why you think that.