Canon is likely to announce a new 200-600mm f4.5-5.6 super telephoto zoom lens in early 2018 according to Canon Rumors. It is expected to be announced at CES 2018 or CP+ Show in February.
There are a few more lenses lined up to be announced by 2018, viz. the EF 50mm f/1.4L IS USM Lens, EF 135mm IS USM Lens and EF 24-70mm f/2.8L IS USM lens. Stay tuned for more info.