Is the venerable 135mm being replaced with an IS version? Any word on its replacement? Is a replacement even needed other than to take more of our money?
Sorry to be the Grinch, but no chance of that anytime soon or even midterm.
Because in terms of satisfaction this lens sits at the tippy-top even in its current form. And Canon has a long long list of must-do things before they start to tinker with things that are highly regarded for both quality and value for the price.
As for IS, as I said earlier in a 135L thread, "Why paint the peacock?"