Gold & Silver were originally going to introduce a Light-type. I had kind of suspected this as a kid, and the Spaceworld beta contained evidence that got me thinking about it again.
I did some digging and I haven't really found anyone making this claim. I can't find any autistic poketuber clickbait about it, and I haven't seen it come up in those dogshit threads where you argue about whether Ash was secretly in a coma. Since this idea doesn't seem widely discussed, I'm going to make a few posts and try to build the best case I can for it.

Light-type was going to be a counterpart to Dark-type. Simple as.
It was always a little strange to introduce Dark by itself. Light was likely going to be retroactively applied to certain 1st-gen Normal types, and some of the decisions were probably encompassed by the Fairy-type rework that we eventually got.
>If Light-type was planned, why doesn't it exist in the Spaceworld beta?
Because it was never finished. Normal-type (and sometimes Psychic-type) were likely being used as placeholders for it while GF made up their minds. I have a strong argument for this and I'll get into it shortly.

A scrapped Light-type becomes apparent when comparing the Spaceworld beta to the finished games. Not only can it be seen in pokemon that were cut entirely, it can also be seen in pokemon that made it into the final game: some changes between the beta and the release suggest the abandonment of Light-type, while other designs were unchanged, giving G/S a weird little group of misfits.

The best jumping off point for the scrapped light/dark concept is Umbreon & Espeon. Umbreon, a Dark-type, evolves at night - simple enough - but Espeon's purple psychic design had absolutely no relation to evolving during the day. While still Psychic-type in the beta, Espeon was this hideous bright yellow. Clearly something prompted the redesign, and Espeon is not the only pokemon that underwent this type of change.