Alright, after your input I decided to make a Gengar and, like @Amethystlamuso suggested, I made it a side evolution, the one where Gastly didn’t fulfill its psychopomp job and rot as a consequence turning it into a Fairy/Poison type. Just to tell people who didn’t see the other post, Gastly is a psychopomp that guides souls to the underworld and only appears where there’s death.
And now that I gave it more thought, I kinda know how would it evolve:
Haunter: Level 20. You have to go to a graveyard, make one of your “non ghost” pokemon faint and then release both Gastly and the other mon. After 3 days, come back and Gastly will appear as Haunter in a random encounter. You would be able to catch it with any pokeball at first try. The other one will be lost, sadly. 🙁
Gengar: Level 30. It will evolve normally but its nature will change to Vengeful or something of sorts.
PS: You guys really like Gengar, huh? XD I never thought that would be the main issue with the design hehehe. Point taken.
by Rocazanova