Agents of Muscle Toon Network has experienced that it's not the first time Disney has worked out concept idea for muscular female characters in Disney series and Phineas and Ferd is no exception.
The episode was titled: Debt of Phineas and Ferb. Thought a series of events, Candice and her mother got muscular and have to pay up a debt Phineas and Ferb accumulated from all their summer by entering a contest for a price money.
Of course their mother still unaware of Phineas and Ferb's antics.
The idea was scraped, when the writers couldn't solve how to turn Candice and her mother back.
There were other female characters planned, but only able to find this model sheet so far.
Artist note: This was intended for someone, but showing it around, the same problem always pointed out and dropped the idea completely. Still, found it on my USB, so might as well post it.