Wow, dtbaker did a pretty great job answering those questions based on a quick phone call a few days ago 
Here’s a first round of answers to the main questions:
Will you develop a custom CMS or is it WordPress?
It’s a custom CMS and hosting platform. After looking at what was available to us, we decided this was the best way to meet our requirements. At this stage, it’s a React web-app backed by a JSON API served by a Ruby on Rails backend. Theme authoring doesn’t require Ruby.
Will Envato Sites support plugins?
Add-ons and extensions will likely be an essential part of an open, community-backed product like this, but it’s not a key focus for us yet.
Can an author with no development expertise can join?
You will need to know HTML & CSS for sure. You’ll need to be familiar with responsive design techniques and have a great eye for design as well. You won’t need experience with programming languages like PHP or Ruby. We’ll provide more information about the theme platform as we progress through the beta, as it could change over the next few months.
How will it work with the existing templates & themes?
Themes need to be built specifically to work with the Envato Sites platform. Themes can be designed from scratch, or authors can port over one of their existing themes. Most of our initial themes will be Envato Sites versions of existing ThemeForest themes, but we’d love to see a huge range of Sites-first themes.
How will authors be paid?
We haven’t made any final decisions yet, but sharing the subscription revenue in some way makes a lot of sense. The buyer is getting ongoing value from the theme and the platform, requiring ongoing investment from both the author and Envato. We’ll continue to consult with authors and find the best balance for everyone.
When will it be released?
It’s too early to predict this. The first few invites have gone out and we’ll gradually invite more people until we think we’re ready for a full launch.