How did humans evolves?
Where did Pokemon Trainers come from?
Years and years before humans evolved...
You are a young Machop who encounters and battles a wild, maybe Alolan, Exeggutor.
After the battle the Exeggutor drops an Exeggcute egg, Exeggegg.
Upon picking this up, you evolve into Exeggchop, probably Fighting/Grass.
Exeggchop is the first "pokemon trainer".
Exeggegg's desire to be surrounded by 5 other Pokemon give the new evolution the CATCH ability.
2ish story lines
First story line in the past living life as a pokemon
Second story line in present day after revived from frozen iceblock
-New feature, your rival can CATCH you and you can ESCAPE rival during rival battles
-Can catch pokemon in trainer battles
-First trainer pokmeon
-Second evolution of Exeggchop, Machoke-esque Machamp evolution of Exeggchop
-Since power belts have not been create the Exeggegg saps the Machoke's uncontrollable power and advances the evolution forward to Machamp
Primal Legendary Beasts
-I have a very specific idea
Baby Kangaskhan (Cubone Theory)
"Primal" Bulbasaur and evolutions
-Politoed? and Oddish co-evolution
I've got a bunch of plot line ideas and a dark twist for the end, but this is the basic idea.