I don't know if anyone markets one, but a large frame bag that is quick to put on and off, and is a regular backpack off the bike. I jury-rigged my own, and it was so easy and works so well that I was embarrassed that I put it off so long.

I think that a frame bag has better aerodynamics than panniers or milk crates, and better balance than wearing backpacks.

Putting a briefcase attachment for panniers would be overkill and a bit silly in my opinion, because the design and fashion factors for briefcases are incompatible with pannier function. Better if you could put a briefcase IN the panniers.

Another one. I've said this before when this question comes up and there ARE solutions but none really satisfactory. I want a rear-view camera showing the image clearly on a largish screen on my handlebars, and not an uber-expensive top end bike computer gadget. I did rig one up from an auto backup-up camera, but it was too difficult to see in direct sunlight, and hard to make out the image sometimes. A good camera with just the right field of view, transmitting to a standard tablet would be ideal. Bundled with the quick release mount with shade/illumination possibly, and tablet app.

And finally, a new one. An idiot light or audible hooked up to a detector which can notify me in several defined situations. The detector must be able to measure the approximate distance, size, speed and direction of travel. That's the catch - a hundred feet range at least, and better a couple of hundred is beyond the capacity of inexpensive off the shelf pieces like sonar range detectors. Briefly, I'd be alerted when a motorcycle or larger is approaching (almost) directly towards me at 10mph relative velocity. In my lane yes, next lane, no. And when any object is behind me at about the same speed, with say 30 feet. Keep it simple, just a tone for each situation, or an LED or blinking icon on the tablet.