Give me a 135L f/1.8 IS. The current 135L is already perfection, but IS and f/1.8 would only make it irresistible.+1
How do you improve on the 135 f/2? The only meaningful way forward is 135 f/1.8is. It's almost certain to be bigger. But it's unlikely to be such astonishing value as the current lens; it could not sell anywhere near the price of the 135 f/2 which is easily found for under $1k new.
IS possibly. BUT, it would cost close to 2K$ and ... who would dare to sell his/hers own superb existing 135mm copy if Canon were to stop its production. I wouldn't ! In addition 1.8 is too much. I cannot find a real reason as it would skyrocket the cost...