Addition of a tab slider. It would make it faster to click in the slider, or drag the slider to go to the desired tab.
I'm not sure about a tab slider, but is it possible to provide a choice of mouse wheel + keyboard mappings? Mouse wheel behaviour is configurable (or at least on/off switchable) for the viewer, but it would be nice to see this extended a little.
On my system it seems that Wheel scrolls the active lister if the mouse is within the lister area, Alt+Wheel enforces active lister scroll regardless of pointer location and Shift+Wheel does back/forward (is this documented? is it all even DOpus controlled/specific?). It would seem logical to offer the choice of Up/Down or Back/Forward, and switching tabs would also be useful on (say) Ctrl+Wheel in a similar (but faster) way to Ctrl+Tab.
FWIW, I've got a mouse with a second "wheel" on the side. (It's not really a wheel, but a rocker switch you can push forwards and backwards, as well as push in.)
Unfortuantely it's a Logitech mouse. Great hardware but their SetPoint series of drivers are not exactly wonderful. (Bring back MouseWare! 
) You have to hack about with XML config files to allow yourself to do anything remotely useful with the sidewheel (assuming you're like me and don't find it useful to use the wheel for zooming or switching applications).
It you do hack about with the XML, though, or are using another mouse that's more easily configured then you can do what I've done. I've assigned Ctrl-PageUp and Ctrl-PageDown to the rocker's forward/back functions and Ctrl-F4 to its click function.
I had to add the Ctrl-PageUp and Ctrl-PageDown to Opus, since they're not the next/prev tab hotkeys by default, but that's trivial.
Now I can switch between tabs in Opus, Firefox and Excel using the mouse. I use it much more in Firefox than Opus to be honest but having the same behaviour in all programs is good.
Of course, this isn't much use if you don't feel like spending $6m on the most fancy mouse. 
Assuming you are talking about the Logitech MX here, any tips on how ones does hack the XML to improve the operatability?
You're in luck as uberOptions has been updated for SetPoint 3.30 so you can let it hack about with your config files isntead of doing things by hand.
Way cool indeed installed uberOptions now – plenty with as you say!