Salkinds initial plans for a 'scifi' Superman III
from trivia
"According to Ilya Salkind, an earlier version of the script included the comic book villains Brainiac and Mr. Mxyzptlk teaming up, and Superman meeting his cousin, Supergirl, which would lead to the potential Supergirl spin-off"
Brainiac would've been a great villain, going back to the opening of the first film, Krypton's destruction etc, offering a potential reason for that. and Supergirl introduced as a supporting character would've been a fun element (potentially leading to a spin off film if popular enough), Could imagine something like Brainiac (Chris Walken?) arriving earth corrupting superman/turning him evil, supergirl (Slater) having to fight him and save him from the dark side, then them teaming up to defeat Brainiac for the finale. That Scifi element that was present in the previous 2 films. theres not really there much of it in III. The Ultimate Computer was like a real world version of Brainiac with superman turning bad - Bizarro (with Webster and his minions obvious stand-ins for Lex and Otis/MissT)
Had that been III could've been a hit on the scale of I/II leading to the Supergirl film (more anticipated after III) then Salkinds continue with IV, V etc as many as Reeve wanted.. share