Buy the Mark I ASAP. That will ensure that a new one is announced 31 days later, just after your return rights expore
The lens has been rumored for 7 going on 8 years now. When Nikon finally updated their 80-400mm to a high quality $2700 lens, many thought Canon would quickly up the ante as well as the price. If there is going to be one, history has shown that there are no leaks. Digital cameras get leaked because a lot more people know about them. Lens announcements usually come as a complete surprise.
"..Nikon...high quality $2700 lens......"!! An overly expensive lens with a p**s poor tripod mount and according to photographylife review not particularly sharp at 400/5.6.
It would be nice to think that the new Canon could be a scaled up version of the the 70-200/2.8 ie 140-400/5.6 with internal focussing and so sharp that a 400/5.6 IS prime would not be necessary!!