This is a gist to track the feature development for Sinatra 2.0.
Keep an eye on the CHANGELOG which is updated as features and bug fixes land. We generally try to keep this limited to "major" changes to cut down on cruft.
At the moment, there's a milestone for 2.0.0 on the issue tracker. But we'll try to land as many features as we can in the next release. No promises.
As I mentioned in the blog post regarding Sinatra 2.0, the largest changes are just dropping support for older versions of Ruby and Rack.
Although using Mustermann the new default routing engine is probably the second biggest change, it's mostly an internal.
This is something that isn't exactly documented, although @tenderlove has spoke a few times about his changes to the Rack request and response API.
There have been a few changes internally to Sinatra in order to support the latest version of Rack, but otherwise we are testing against Rack master so any breaking changes should be caught by CI.
With that said, I (too) would love to see the latest changes in Rack 2.0 documented :)