Spring Loaded tools
Over in Photoshop, it's possible to hold the single-letter shortcut for activating a tool, and to then release the key to return to your previously-used tool.
In many ways, this is not useful in Illustrator, but in several, it's invaluable. The Zoom tool, in particular--I'd like to be able to hold Z, adjust the zoom, and release Z to be back on the selection tool (or whatever i was previously using).
The Hand tool already works this way: Take out a tool like Selection (v), and then hold spacebar to get the Hand tool. Drag the canvas around, then release Space to return to using the selection tool.
This "spring loaded" behavior is great in the hand tool and could be similarly useful in the following tools, where a quick "adjustment" with the mouse would naturally be made while the tool key is held.
Additionally, these functions would benefit, but are not single-key shortcuts by default:
Rotate, Scale, Shear
Bloat, Pucker, Warp, Twirl, Scallop, Crystalize, Wrinkle
There may, of course, be others, but these are the main ones I can think of that do not rely on a modal settings box or panel to do their work, and which primarily involve visual adjustments by clicking and dragging with the mouse and without (usually) needing modifier keys. As such, each would benefit greatly from being "spring loaded."
(Symbol Sprayer and its variants might also benefit, but generally require modifier keys)
Kasey Dutton commented
I use a drawing pad and I loose the pen tools connection to the anchor every time I need to zoom. It works with the mouse scroll wheel but not with the pad. This would really speed up my work flow.
Can we have a toggle setting in preferences for the 'z' zoom tool key. Works like this. Hold the z key down to activate the zoom, drag or zoom with the mouse to the required size, let go of the zoom key and it goes automatically back to the tool you were using before zooming. Without having to press any other keys like cmd+space on mac. Saves time having to move to press 2 keys to do this and also removes a press on another key to get back to the tool you were previously using (based on the way it currently works).