He didn't plan out the entire Dragon Ball series though. He made it up as he went along. There's several arcs which should have ended the series but it was continued due to popularity.
So we know that Piccolo and Kami are aliens but they were not originally intended to be.
Right, but that's how just about all stories are told. Very seldom does a writer have a book completely pre-written in his head. There are always new ideas coming in and changes that are made. That's how television shows, books, and original film franchises are constructed.
Rather than thinking of it in the terms that they were never meant to be aliens, I think Toriyama would prefer for fans to be in the dark regarding Piccolo's true origin, just as the character was. The same of all this could be said about Goku. Nobody had a clue that he was from another planet. He was just found in the woods by a kind old man. Toriyama developed his origin later in the Saiyan Saga, but it was an origin that he intended to be canon, even during the time we weren't aware of it in Dragon Ball. So we could go back and watch DB, and that origin is still of canonical merit.
I was introduced through DBZ, like 90% of the fans, but after having seen Dragon Ball, neither series was the same to me. They were each completely different series apart, than they were together.
GT is the only series I would view as a separate entity from DB/Z, simply because it makes so little sense, and the story is utterly riddled with plotholes. Not to mention the fact that Akira Toriyama had virtually nothing to do with it. What a mess TOEI made of his mythos in an effort to continue making money.
I think the combination of mythology and sci-fi is great, I hope that this shows up in the film because I think it's a very important part of the series.
This is what, I think, made Dragon Ball Z vastly popular among the general public. Dragon Ball had a strong fanbase, but it's clear that Toriyama's story tripled in popularity once the science-fiction element was more heavily expanded upon.
I don't want it to be like Goku is a teenager that gets picked on in school, but he finds out he has super powers and Piccolo comes to Earth in a space pod and is like "Alright I'm taking over!" Oh, you're an alien too, let's team up. No? Ok, prepare to die!"
God, I hope not. I can't imagine any fan who would want something even remotely close to this.