This is a good topic; I watched a twitch streamer recently and was floored they were clicking through pve content at max level.
First I found it game changing to swap ctrl and caps lock. Corsair keyboards can do this easily. Either way swapping capslock with the control modifier will open up things, especially for anyone with smaller hands. So that leaves you with two modifiers in easy reach - shift and control. Movement is done via wasd however playing with a mouselook toggle is also a god send. There's an addon at curse that allows for mouselook with a simple keybind to allow for a toggle. I use the extra mouse buttons for this. WoW also has a keybind option for "interact with target" which essentially allows you to loot without the right click.
Here are my keybinds across games and all of my characters.
`, 1, 2, 3, 4 gives you five buttons plus with two modifiers, already you're at a possible 15 keybinds.
q,e,r,f, are another four plus your two modifiers.
z,x,c,v will add four more buttons plus two mods.
spacebar+modifiers does not interfere with jump and is great for auto run or mounting, whatever.
Two mouse buttons + modifiers
This is ultimately 3 bars worth of keybinds and all classes are reduced down to 1 bar worth of combat abilities used consistently.