Pokemon Red & Green underwent a lot of changes during development, and there's evidence to suggest that Erika was originally meant to be a Ghost-type Gym Leader.
There have been a number of Pokemon leaks over the past few years, including beta builds of Pokemon Gold & Silver from '97, which included tons of unused Pokemon designs. There have even been leaks for Pokemon Sword & Shield, showing Pokemon that never made the cut during 'Dexit. The leaked version of Pokemon Red & Green might be the most interesting of them all, as it shows a vastly different vision for the Kanto region. One character that fans suspect was changed during development was Erika, the Grass-type Gym Leader of Celadon City. There are elements of Erika's design, as well as evidence from unused concept art and the leaked beta of Pokemon Red & Green, suggesting that she was originally meant to be a Ghost-type Gym Leader. This theory was outlined on the Pokemon Conspiracies Reddit page.
The Design Change
The majority of people who played Pokemon Red & Blue in Europe and North America wouldn't have noticed anything off with Erika's design, but there is something strange about it. Erika is wearing a yukata, which is a type of kimono worn in Japan. Traditionally, people wear their yukata with the left side folded over the right. The exception to this is when bodies are being prepared for a funeral. In those instances, the corpse has their yukata worn with the right side folded over the left. In Pokemon Red & Blue, Erika is wearing her yukata in the dead body style. This was changed in Pokemon Yellow and all subsequent appearances.
The Gym Leader Shake-Up & Lavender Town
We know that the Gym Leaders in Pokemon Red & Blue underwent a shake-up during development, as was originally hinted by concept art that was shared to Twitter, and later confirmed by the Pokemon Red & Green leak. In short: Giovanni was originally a child and the first Gym leader in the game. Brock's design was radically different and he resembled Krillin from Dragon Ball Z. Blaine resembled an army general instead of a scientist, which is why he looked so different in the Pokemon anime. Sabrina was originally the final Gym Leader instead of Giovanni. The map of Pokemon Red & Green changed over the course of development, and the Gym Leaders were shuffled around to accommodate the different positions of the cities.
Erika's concept art is interesting for a couple of reasons. For one thing, her eyes are open, and they have a glassy expression to them. It's easy to presume that she is tossing the Poke Ball in the air in her Pokemon Red & Blue sprite, but the concept art suggests she might be floating it with supernatural powers. Erika is also listed as the fifth Gym Leader (she's the fourth in the final version of the game), and Lavender Town was listed as the fifth city in the original concept art for the game.
This evidence suggests that Lavender Town could have had a Ghost-type Gym at one point during development and that Erika was the Ghost-type using Gym Leader. It's easy to imagine the Channelers in the Pokemon Tower as the inhabitants of Erika's creepy Gym.
Did Agatha Cause The Change?
This raises the question of why Erika had her type switched? One possible reason for this is Agatha of the Elite Four. Agatha originally had a drastically different line-up in the Pokemon Red & Green beta, as her teams consisted of a mix of Eeveelutions, Tangela, or Tauros. She didn't become a Ghost-type trainer until later in development.
There was a lack of Ghost-type Pokemon in Pokemon Red & Blue. The only three Ghost-types in Gen I were Gastly, Haunter, and Gengar. Meanwhile, there were 14 Grass-type or dual Grass-type Pokemon and most of them were fairly common. It's possible that the developers decided to shift the rarer/stronger Ghost-type Pokemon to Agatha, as she already matched their creepy design. Erika was then given a Grass-type Gym in Celadon City. Her corpse outfit was switched to a summer theme, and her gimmick was switched to having a Gym full of female trainers. The idea of a Gym Leader with supernatural powers was also used with Sabrina. It's possible that this gimmick was switched from Erika to Sabrina, as it made more sense for a Psychic-type Gym Leader to possess these abilities.
The first Ghost-type Gym in the Pokemon series was the one in Ecruteak City in Pokemon Gold & Silver. Morty was the first Ghost-type Gym Leader, and it's interesting to note that he only uses Gen I Pokemon, even though Pokemon Gold & Silver introduced more Ghost-type Pokemon. It's possible that Morty used the same line-up that was planned for Erika in Pokemon Red & Green.
However, until official confirmation, we may never truly know.
Source: Reddit