Secure Desktop Mode
Vivaldi seems to want to look for gimmicks in order to help promote the browser, so here's an idea that might attract a lot of attention.
A way to spawn what is effectively a guest window session, but in a Windows 7+ Secure Desktop.
A secure desktop, is what you get when you're running UAC at the higher levels - where the window isn't actually shown on your real desktop, but on an alternate one. My Password manager KeePass can do this, the intent being that it helps guard against most keyloggers (not all). The limitation about most seems to revolve around needing to be able to interact with other applications, which Vivaldi doesn't need to worry about.
I got the idea after screen-surfing someone who was working on their laptop. They were doing something, and their Internet security software offered them a way to open a "secure browser", which from what I understood also seemed to use a secure desktop. Whether it actually did or not, who knows.
Aimed at users who want to complete a particular interaction with a website in the highest security. It would prevent access with other apps, so isn't intended as a normal browsing mode, but say you wanted to more securely do your internet banking,this would be a possible way to give more assurance of security.
I am not incredibly well-versed in how secure desktop is managed, so the whole thing may well be security theatre. But if its good enough for a password manager, I'm inclined to think there is some benefit to it.
If I can evangelise at the last possible second: Don't disable your UAC.