The key factor in the character's appearance was that Dave Filoni wanted it to be in line with the Prequels rather than the original trilogy.
Darth Maul vs Obi-Wan | Star Wars Rebels | DisneyXD
The Inquisitors are perhaps one of the most menacing entries Dave Filoni has dreamed up for the Star Wars universe. Essentially delivering an army of Sith without breaking the dreaded rule of two, the Inquisitors were the perfect addition to the franchise to address the issues of the surviving Jedi and give them a narrative challenge without seeming weak or circling back to Darth Vader.
The Grand Inquisitor was a major threat to the characters in Star Wars: Rebels, and even made an appearance in Obi-Wan Kenobi. The character went through quite a few iterations before his appearance was finalized for the show, as revealed in Star Wars Insider Magazine by Dave Filoni himself.
Dave Filoni revealed that the Grand Inquisitor, depicted as a Pau'an, was originally intended to be a Chagrian, the same race as Palapatine's aide, Mas Amedda. When designing the character, Filoni felt it was important to play with how a villain looks, which he always considered a challenge. Citing the example of Vader and Maul, Filoni revealed what was going through his mind to Star Wars Insider:
