The second analog slider included in a revision could work. It just requires developers to program 2 control methods (one for 3DS, another for 3DS+) which some already do on other hardware, namely the Wii. Some developers have multiple control options without different controller options such as Resident Evil 5. Speaking of, The Mercenaries 3D would play much better with that other slider for those wondering what games would benefit from it. Also, in-game camera.
Adding a second slider isn't completely out of the question. DSi included 2 camera, an SD card slot, more RAM, faster processors, and larger screens. Granted, few developers took advantage of all of that but there's precedence for adding functionality. Will Nintendo do it? Perhaps but I'd personally bet against it. I don't think a second slider accessory is even possible.
The biggest problem would be how pissed off current 3DS owners would be should a revision fix one of the orignal's glaring omissions, not to mention the possibility of fixing ALL of them such as battery life. Does Nintendo try to compensate them again? I had always planned on buying a revision so I wouldn't be upset at all. The free games and sudden price drop just means my brother gets a hand-me-down 3DS once I transfer the downloaded games over.