I strongly doubt Shultz ever had any plans to ever put Charlie Brown and the Little Red-Haired girl together. First off, she was an unseen character. Second, Shultz said himself that you can't create humor out of sheer happiness, Charlie Brown is the fall guy and it stays that way, even though sometimes good things happen to him, but they're never that major.
In fact, in the old TV special You're in Love, Charlie Brown, which was about Charlie Brown trying to get the Little Red Haired Girl to notice him (and possibly like him), he ends up finding a letter from her saying that she likes him in the end, but it's the last day of school. He then thinks that he may now have a chance next year after all of his time spent as a "nobody", but soon realizes how long of a wait it'll be until next September.
So basically, Charlie Brown's streak of being a loser never ends, but at least he's a likeable person. That's the way Shultz set it out.