So when you create larger site or a small one that demands user login features the Preview mode won't work at all in Wix. So this means that you have to publish your site and then test it if it works. This is not working together with clients where you do need to make a new function or update something and the system in already live.
My suggested feature would be...
Preview As... would be a button on top of the editor if you have activated the member app. When you click that you get a dropdown in some way with the sites members in it or the sites roles. By doing this you would be able to Preview your site logged in as another user and you can test features.
Or... the most professional way and this would of course only be available for Premium Sites.
You have PREVIEW, you have PUBLISH and now you have STAGE.
What happens when you press STAGE?
Your preview version of code and Data Collections gets published on https://staging.yourpremiumdomain.com/ and when you hit that site you will need to login as an Administrator or any other role that have the permission to see staging site.
On this site you can test it all, work with and show the client and when they are happy with the new version of the site you just hit PUBLISH and the pages, code and so gets published and it will then use the data from the live Data Collections.
The staging feature would be a killer feature for Wix and appeal to pro users that are used to work in this kind of setups.