Well, Craig Bartlett said the spinoff would have been slightly darker and more mature than the original series... apparently, in another twist, Helga and Lila would have actually become best friends, but I believe it was said because they realized they could relate to one another and had a connection between them, considering both of them have hidden dark sides to them stemming from less-than-ideal childhoods (Helga's issues with her family, and Lila and her father once living in absolute poverty).
In addition to that, Gerald and Phoebe would have finally officially hooked up; I think Craig even said they were going to be the "cool couple" at school, so yeah.
Actually, there's a fan-animated trailer on YouTube, I wish I remember what the title of it is so I can look it up, but it was pretty well animated and kind of tugged at the heart strings with Helga and Arnold trying to meet up with each other one last time before he relocates.