I haven't really seen this mentioned anywhere, but I think Sabrina was intended to be the final Gym Leader in Gen I, and that Giovanni was a late addition. Like take a look at the evidence.
>The Japanese name for the Marsh Badge is the Gold Badge, and of the 9 original badges that went unused, the Gold Badge was the last
>Saffron City is the final city by index number in Gen I
>Saffron City is the final location choice when using Fly in Gen I
>Saffron City is Tokyo on a real world map of Kanto, the capital of Japan
>Saffron City is blocked off for a fair portion of the game, and it's possible that you weren't originally supposed to enter it until the endgame
>In Ken Sugimori's concept artwork for Red and Green, there's an unused Gym Leader and numbers indicating everyone's sequence. Sabrina is shown, but her number isn't, and it isn't 1-7
1 - Unused boy (presumably Viridian City)
2 - Brock
3 - Misty
4 - Lt. Surge
5 - Erika
6 - Koga
7 - Blaine
>Psychic-type Pokémon are the strongest in the game by a good margin, and would make for a fitting final Gym battle