What if there were certain Pokemon that if you capture them they stay with you for a long-term period or even for the rest of your life regardless of if you want them to or not.
I'll use Gengar as an example, Gengar is a doppelganger that becomes or hides in a person's shadow feeding on their life-force, but what if it actually stays in a person's shadow and never leaves? That person wouldn't be able to get rid of or abandon it because you cant leave your shadow.
Organizations like Team Rocket and team Plasma could not steal these Pokemon either, they're impossible to steal since they cant leave you; they would have to kidnap you in order to kidnap the Pokemon.
If there were some Pokemon with qualities to Gengar where the trainer is bound to them, then they'd never end up like Ash's Charmander and Tepig since where you go, they go.
And also in the game series, there could be certain Pokemon where if you capture them, you cant release them back into the wild, and some of them would be even more extreme about it, you wouldn't be able to deposit them into the PC, or even switch them out in battle, they'd have a permanent natural "Mean Look" effect so you'd have to be extra careful about placing them first on your team. You couldn't even trade them to another player(they'd refuse the trade). But, you'd get some type of warning of this right before you decide to throw the Pokeball. Or possibly even have to sign a contract with some(like signing a deal with a demon or the devil).
What do you think about this? :)