Until I began writing this up, it hadn't really hit me how few telephoto zoom lenses Panasonic offered. Out of the 26 lenses it released since 2012, only 6 extend beyond 100mm and none of them are primes or really could be considered pro-level.
Then I looked at Olympus' options, and they weren't much better. While mounting on a Micro Four Thirds camera essentially doubles the focal length, making that 100mm lens the equivalent of a 200mm lens, the lens offerings for that mount are generally kind of cheap or old.
Going for the birders, Panasonic's kicking it up a notch with its latest, the Leica DG Vario-Elmar 100-400mm f4-6.3, a lens with an effective focal range of 200-800mm. What raises it slightly above the crowd is a 9-blade aperture (for rounded out-of-focus highlights) and a dust-and-splashproof design.
It's a definite improvement on Panasonic's older 100-300mm model, with significantly better optics for sharper photos and modernized hybrid optical image stabilization designed to work with supporting in-body stabilization cameras like the GX8. It's also got a linear motor, necessary for remaining quiet when recording video.
Plus, it's faster than the alternative, Olympus' 75-300mm f4.8-f6.7 II, which lacks image stabilization so isn't great for owners of pre-2015 Panasonic cameras. It focuses closer 1.3m/4.3 feet compared to 1.5m/4.9 feet, but it's longer by about 45mm/1.8 inches and heavier by 465g/1 pound. It takes a bigger filter too, 72mm.
It doesn't come cheap at $1,800, though that's a pretty typical price for this type of lens. You should be able to buy it in April.