I'm sure this has been discussed before, but with a quick search I found nothing.
Vivaldi is known for being ultra customisable and for power users, so given that a fair few people probably use a new MBP (I got mine this week) it would be a great addition for Vivaldi.
Safari uses it for quick tabs/bookmarks which looks great, that would work well. But I'm sure there are lots of other possibilities and if it could be highly customisable that would be great!
You forget that only a fraction of users is using osx, and another fraction of them is using the new macbook pro. The only browser besides safari to implement it thus far is opera, afaik even chrome doesn't do it (yet?). It's simply not a priority.
Also depends on how much work it is. Try and post it in the feature request thread.
Fear not! You can use BetterTouchTool to customize the toolbar in any way you may want.
Google developers have added Touchbar support to Chrome/Chromium 58, which is still in beta.
As you can see, some of the bugs have been tagged as Stable release blockers so its looking very good that the Touchbar will be supported in Chrome 58. However, the Touchbar code also integrates into Chrome's code for search and bookmarks; Not having any insight into Vivaldi development or product direction, I don't know how challenging it will be for the Vivaldi devs to add the necessary hooks into their own code, but at least there's a foundation for them to build on.