I just want to suggest you either 1) Make a plugin to connect Pro Sites with Gravity Forms to handle user registration or 2) Make your own plugin that allows a site owner to create a custom registration form. There have been lots of requests for this in the forums, as well as lots of people saying they don't like the default WordPress registration process. Edublogs doesn't use it, and even WordPress.com doesn't use the default WP registration process because it doesn't make sense. There are too many steps and it's just clunky and not user friendly.
Yes, I already know about this:
That's a great effort to try to connect the two. However, it's not full integration. It isn't set up currently for subscription/recurring payments. I'm using Stripe and I can use the Stripe add-on for Gravity Forms but it doesn't tell Pro Sites that the customer has a recurring subscription. You can't use the built-in Pro Sites account menu to alter a subscription (upgrade/downgrade/cancel). I also have my doubts about whether it will work with the Affiliate plugin. In other words, it's not a full integration.
I think the ideal thing would be to have a plugin that acts as a Gravity Forms add-on, like other GF add-ons, that lets you create a "feed" and map different fields on your form to the required Pro Sites fields and and registration options.
What I want is a simple, one-page registration form that includes payment information if the customer selects a paid site (Gravity Form's conditional fields are perfect for showing/hiding fields). But I also want some additional contact information from customers - phone number and address.
I could make a custom wp-signup.php but then it will get overwritten on a regular basis when WordPress updates. That's not a good solution.
I also hate the way customers have to manually log in to their site before they can even see the option to upgrade their site. It just doesn't make sense. I've read in the wpmudev forums of lots of people complaining about this. It isn't the logical flow. It's not what I'd expect and it's not what the customer would expect. It doesn't make sense. I know lots of site owners are really disappointed after installing Pro Sites and trying to go through the signup process and see how it works. I know I was.
We really need a better solution!