Extenders equipped with a switch to flip them into / out of the optical path (like the built-in extender in the 200-400 f/4 L IS).
Sure, they'd be larger and more expensive than the current models, but the technology is now proven, at least with the 1.4x.
When I replaced my 70-200/2.8 L IS with the II version recently, I also sold my 1.4x II, which I'm not replacing. It's just too much hassle to insert and remove. For the extra reach, I'm hopeful for a 100-400 II this year, with reviews as glowing as the new 16-35/4.