As for heroic scenarios, players will be able to participate in that content as well -- there's a new section in the scenario queuing system that will allow players to queue for heroic rather than regular scenario content. EU players can leave feedback on the official forum thread, and presumably US players will have their own thread for doing so as well. If you've been itching to find out more about upcoming 5.3 content, this is your chance to take a peek and give your feedback on the content.
If you're interesting in participating, you can download the PTR client via your account management page on Battle.net, as well as copy an existing character over to the PTR. Blizzard seems to be looking specifically for people's reactions to the relative challenge of the scenarios, as well as feedback regarding the new bonus objective system. Keep your eyes peeled, we'll have more information on all of the new scenarios once they are available and open for play.