First off, I apologize if this has been suggested before, but there haven't been any changes to it for years now.
Allow us to change the position, size, speed of experience drops and location of our total experience bar. Right now everyone with above 1B total XP will only see the last 9 digits of it. Displayed like this:
I have 1.3B total XP, but it only shows 326M. Right now it's only possible to reset your total XP.
Obviously, this is not something that needs to be fixed right away, but it would make the game look much more nicer and up to par with OSRS.
In OldSchool, you can easily configure your total XP bar. You can change the colors of your experience drops, set goals, add a progress bar and much more. Like this:
There are also other interfaces that could (and should) be made up to date with OldSchool, but I won't be going into too much detail with these.