[@Reflection] I've had a few thoughts, but I can't really decide which ideas to keep and which to discard. Like, here's a quick list of concepts to give an idea of what's going through my brain:

1. Earthling/Human that trained under Son Gohan. Since Goku never showed up it stands to reason that Grandpa Gohan never died. As such it sort of frees him up for virtually any kind of backstory. Ranging from him being a kind old uncle to the character or maybe just Miyagi-esque friend and mentor and nothing more.

2. Earthling/Human that is part of a group called the Galactic 100. They're basically like the Galactic Patrol, but as their name suggests there's exactly 100 members, each one representing a different planet. This one is fun but I'll probably not go with it, since the whole concept will just sort of lose all its meaning after the Frieza Arc and everything goes back to being Earth-based.

3. Earthling Half-Saiyan. I'm not entirely sure if Half-Saiyans are even allowed, so I'll probably scrap that element if that turns out to be the case and just have him be pure Earthling. This one would be an orphan that was raised in Nam's village, and actually trained under Nam himself as well. As for the Half-Saiyan part, well, I would borrow the Galactic 100 concept (or at least Galactic Patrol) and use that as a way for his Earthling parent to have met a Saiyan and eventually marry them. If I do go this route I'll include a bit where as a baby he was born on a ship that fell under attack by the Ginyu Force. His parents would have managed to get the infant into an escape pod and shoot it out into space before they were wiped out by the Ginyus. Since Planet Vegeta is no more they send the child to the planet of his other parent, Earth. The escape pod crashes in the desert and the baby is taken in by Nam's village. Or, if I scrap the Half-Saiyan bit, he'll just be a regular Orphan that was found abandoned in the desert and, again, taken in by Nam's village.