I would love to see an updated mobile UI - primarily taking another look at the bottom toolbar/menu as I think the current icons aren't necessarily the best use of space.
Specifically, I think the back/forward buttons probably aren't that useful for the majority of users as the same actions can normally be performed via swipe gestures, or (in the case of Android) built in navigation in the OS. These buttons could probably be relocated to the secondary flyout menu.
In their place, I think it would be much more useful to have a new tab button, and most importantly a search button - which essentially gives focus to the URL bar.
A lot of users frequently request placing the URL bar at the bottom to make it easier to reach, but Brave implements a search button on the bottom bar (see screenshot), and I've found this has completely removed the need for a bottom URL bar.
I'm sure countless others will disagree with how I'd personally like my icons set up, which is why, like most things - I think the abilty for users to customise the UI to suit them would be best.
We already have this option for the secondary menu, it would be great if the primary UI menu could be customised in the same way.