First I have to start with how much I love workplaces. If extended properly, it can be the feature that brings more people to Opera and make them never leave. It can make opera much more than a web browser.
for each workplace, allow users to add blacklist/whitelist for certain websites. If a user tries to go to a blacklisted (or not whitelisted) website while on that workplace, a page appears informing user that this site is not allowed on this workplace. A button could be added that takes the user to the same website on a different workplace.
Why this feature is useful:
I imagine many users customize their workplaces on a work-related, non-work-related basis. However over the course of the day I find myself browsing youtube for example on my working workplace. This feature will ensure that workplaces are used as intended and help users (or me) to break their bad habits of mixing working with distractions.
I used to use RescueTime which had a feature (paid) called focus mode. when enabled, it blocks entertaining sites for 30 mins. I love this feature and it's very, very useful and effective. My suggested feature can make Opera be used similarly by blocking entertainment sites on certain workspaces.
Finally, I believe that if a user customizes his/her workplaces and builds his black/white list, it would be very unlikely for him to switch to another browser that doesn't have this feature. Moreover, it can make new users come to Opera instead of using a separate software that would provide similar functionality.