Love Vivaldi so far!
I really like the user interface, hopefully you don't change the look. I enjoy the color of the browser changing with the dominant theme of the url. I would like the tabs to be colored, would help when I have multiples open. There are some things in Sleipnir 4 Browser that, you are doing, that I really liked, but what I haven't seen yet is how they do the tab groups and allow you to color/rename them. I think this would be great for your Speed Dial tabs. I am not sure how to explain how Sleipnir does it but it allows you to have Tab Groups (ie Speed Dials) for different types of things, but it allows you to keep them at the top of the page colored and named. So you could have a Speed Dial for Music sites but name it and color it specific to your "Music Speed Dial" I might need to take a picture of the screen to give a better idea. they don't work like speed dial but how it works could be incorporated into the speed dial V has and be pretty cool. It has under each tab group a separate space for open tabs... Let me get a picture a little later to explain. Would be awesome to make Speed Dials work this way.