Firefox is known for having the most extensible browser as far as addons are concerned with their use of XPCOM/XUL. The entire UI and its behaviour can be modified with addons. Chrome does not have this luxury and, as a result, its addons pale in comparison to those available for Firefox. What are the developers plans for Vivaldi's addon model?
This is the only native extension for Vivaldi that I know of - and I don't know if it even works in the latest release.
There's a skin made by community member, widget, over here too.
And there's a whole thread about customizing Vivaldi started by An_dz over here.
People ask about small tweaks they'd like to make to their browser pretty frequently on the forums, and at least one person usually replies with a quick edit they can make to the browser's stylesheet that will have the affect they're looking for. Keep an eye out on the All Platforms subforum, because that's where most customization tips are posted.
As for what the devs' plans are for extensions in Vivaldi's future - I don't know. But I'm currently content in the knowledge that Vivaldi is already highly configurable, and that the developers are keeping the community in mind. I don't know what Vivaldi's future is, but it's gonna be good.