OH MY GOD! Are you serious with this question?!? Or are you just mocking us?
Bring back editing the UI, custom toolbars, custom side panels, plugins for custom side panels, features Opera 12 had, such as Opera's Notepad, UI checkboxes for commonly used settings to be placed in custom toolbars (I would love a checkbox for JS enable/disable and a checkbox for Images enable/disable), directly editable UI colors without the need to search for a theme, smaller UI without margins/padding (90's style), HTML source code live patching with REGEX the way AdMuncher did it, save page link as .url files, a self-hosted sync server that can run on a OpenWrt router with 64MB RAM (no, I'm not talking about a nodejs crap that needs 8GB ram just for itself!), self hosted download manager as linux console app on a different computer (like Aria2, but with better integration), etc. etc.
This list is just what I could think of in 10 min. I'm sure that $$$ budget could build so much more. How about a new browser from scratch?
Edit:
Here's more: embedable engine allowing others to build browsers based on it, framebuffer support like "links -g" allowing it to run in linux console without X/wayland, ability to act as a remote rendering service to allow a very low power device to use it as a remote web rendering server to offload web page JS/CSS workloads (like Opera Mini worked, but with the intermediary server hosted by the user), integration with OS and/or DHCP server to request a new IPv6 IP for each page visited