Fortunately the rumor is the 15-35 F/4L is actually a 14-35mm F/4L! Going to be tough for me to decide if I want to replace my 16-35 F4L with that or the 15-35mm F/2.8L.
My current 77mm filters can be used on my RF70-200mm, EF24-105 and EF16-35mm/4. CPL/6 stop ND and 10 stop ND are screw in and my grads are 100mm wide on 77mm thread.
Moving from my EF16-35mm/4 to RF would mean new filters for >77mm thread. That will be expensive for me. 82mm for the RF15-35mm/2.8 and would be larger if it is 14-35mm.
Unlikely that a 14-35mm lense would accept a front filter thread ie bulbous and need a special setup like the Nikon 14-24mm or EF11-24mm.
I have my Samyang 14mm/2.8 if I need wider but it is mostly for astrolandscapes ie not using filters
I'm sure that a new RF wide angle zoom will be great but it would add another AUD1k minimum for the new filters.... Much more if 150mm grads would be needed.