It's only too soon if you present the characters exactly as they were presented the last time. I"m talking about a darker Green Goblin who looks like he did in the comic book. It's almost a completely different villain if you do it like that, especially when it comes to visuals.
with Otto, I'm not talking about a friendly charming Dock Ock. I'm talking about the arrogant jerk with the horrible childhood.
The low budget doesn't mean too much. CGI is getting less expensive. Look at district 9. That was only 30 million, and the aliens looked completely real. Most CGI costs are when you initially figure out how characters will look, how you will create them and developing the techniques for making it look real, figuring out how they will move ect. They already have all that ready for spider-man. The high wire set ups, ect. Green Goblin wouldn't be that expensive at all today.
And Harry Osborn wasn't really a Goblin in SP 3. He was a guy with gadgets who wanted revenge. He was done well, but he wasn't a goblin. Not in appearance or name even.