Ideas

Extension that does something like this:

def build(*)
  if settings.memcached?
    use Rack::Cache, :backend => :memcached
    use Rack::Session::Memcached
    # ...
  end
  super
end

sinatra-smart-cache: update cache header only if arguments are more restrictive than current value, set caching headers that way for most helper methods (i.e. send_file)

Some verbose logging extension: Log what filters, routes, error handlers, templates, and so on is used.

Form helpers, with forms as first class objects that accepts hashes or something, so the form meta data can also be used to expose a JSON API or similar, possibly defining routes (like "Sinatra's Hat"), strictly using the ActiveModel API.

Extend sinatra-content-for to support Liquid, Radius, Markaby, Nokogiri and Builder. At least the first two probably involve patching Tilt.

Rewrite of sinatra-compass?

Helpers for HTML escaping and such.