I think there is a new sidebar extension with every new version of Opera, having the possibility to add any one of them would sure be really great, this could be done by using the mobile version of a website in order to have something that could perfectly fit in the sidebar, this is the method some browsers use, the only problem being that some website on mobile ask you to download their app.
Another one would be to simply fit the website into the sidebar panel, altho this could create a really messy result, both methods have their pro's and cons.
I believe the Opera team have already considered the option to fit anything in the sidebar, and my theory is that they want to avoid any of the previously mentioned problems, as such they make sure to add one thing at a time, with the benefits of having a perfect integration.
Altho i really really want a gmail integration, was really disappointed with the instagram one 😿
