Tamron working on Multiple high quality Prime lens patent. The Lenses include 300mm F2.8, 400mm F4, 500mm F4 Lens Patent. As we have said earlier also, patent takes two to three years to become a real product. So, it is possible we may see these lenses
Details of the patent is below
[Publication number] Japanese Unexamined Patent
Application Publication No. 2017-223891 (P2017-223891A) click here
[Publication date] December 21, Heisei 20 (2017.12.21)
Patent application title: Optical system and imaging device
Embodiment 3: 500mm F4
Focal length: 485.045 mm
F No.: 4.001
ω: 2.530
Image height: 21.64 mm
Back focus: 125.2621 mm
Lens length: 420.001 mm 
Embodiment 2: 400mm F4
Focal length: 396.296 mm
F No.: 4.0
ω: 2.963
Image height: 21.64 mm
Back focus: 102.3549 mm
Lens length: 332.36 mm 
Embodiment 1: 300mm F2.8
Focal length: 304.799 mm
F No.: 2.8
ω: 3.997
Image height: 21.64 mm
Back focus: 70.1919 mm
Lens length: 298.38 mm
Take a look at more Tamron Patents
Those a very large lenses, much longer than the Canon versions. Hopefully they aren’t this big.