The Lumix GH5 will be the successor of four Micro Four Thirds cameras which have given us top video recording performance in a photography oriented camera body, at a reasonable price:
We have been waiting for a GH5 announcement since the Lumix GH4 was launched on February 7th, 2014. This is unusually long, more than two years.
However, keep in mind that there is a much lower turnover in the camera business now, since a few years ago. People buy much less digital cameras than around 4-5 years ago.
Looking at the announcement timings above, we can see that there are several cameras which are waiting for an upgrade: The Lumix GF8 is essentially the same as the Lumix GF7. So we should expect a less expensive entry camera soon.
Also, the GM-series is getting old, so perhaps we get a new miniature camera. Rumors say that we could get two new Lumix camera models this autumn, in addition to the Lumix GH5.
As for the Lumix GH5, rumors say that it could be announced at the Fotokina trade fair at the end of September, however, without the final product or specifications being available. The camera itself is expected to be available in shops early 2017, which is sad as they would miss the holiday season.
While the GH series has a photography oriented form factor, it is the video capabilities which sell it. The headline feature of the GH5 is rumored to be 4k video recording using the whole sensor area at 60FPS. Presumably, also 50FPS and 48FPS will be available, if the GH5 retains the very useful multi area feature of the Lumix GH4.
Using the whole sensor is a big deal. The GH4, and also subsequent Lumix cameras that support 4K, only record 4K video using a cropped centre area of the sensor, see the illustration below:
This means that in 4K mode, there is an additional 1.3x crop factor. So your kit zoom lens starting at 14mm effectively becomes 18mm in the short end. Getting good wide angle in 4K becomes difficult.
So why is this difficult? Using the whole sensor essentially means reading all the sensor values from the 16:9 crop of the sensor area, and then downscaling. And downscaling is the important key word here, as it is requires a lot of processing power. The GH4 simply couldn't get this done quickly enough.
The GH5 is also rumored to output 4K video at a 60p framerate. If it does, then this would be a very revolutionary feature. Sure, there exist cameras which can do this already, but they tend to be extremely expensive and also large. Only some large pro video camcorders can record 4K video faster than 30p these days.
When I record 4k30p footage using the Lumix GH4, I usually use the image stabilizer feature in Adobe Premiere Pro. This is needed when not using a tripod, in my opinion. In this combination, rendering one minute of video footage usually takes around 10 hours on my laptop computer. Upping the framerate to 60p would probably double this processing time. So make sure you have computer equipment to match the camera.
The Lumix GH5 is rumored to get a 20MP sensor for photo use, perhaps the same used in the Lumix GX8.
It also brings a 30FPS 6K Photo feature, which essentially means that the camera will record a 30FPS video at 6K resolution, from which you can later select the still images you want to keep. This will only give you the JPEG image output, not RAW. This means that the camera must be able to read the whole sensor slightly quicker than 1/30s, meaning that rolling shutter effects will still be a problem, read more about it here.
In body image stabilization (IBIS)
The Lumix GX7 introduced image stabilization based on sensor movements in 2013, a feature previously reserved for Olympus cameras only. However, it was not super effective for photos, and could not at all be used for videos. The Lumix GX85 from 2016 has IBIS, and it works even for videos. So some are now expecting to see this feature also in Lumix GH5.
However, rumors say that the Lumix GH5 will not get IBIS. I guess professional users will have the camera on a tripod or a stabilizing rig anyway, so this might not be a big issue.
No rumors about the price level yet. However, if the Lumix GH5 does deliver 4k60p video output, then the price could be quite high. Keep in mind that pretty much no affordable cameras support this today. Even the recently announced Canon EOS 5D Mk IV, which is Canon's most video optimized DSLR, only goes to 4k30p. And it costs US$3500.
I think the price level could well be around US$2000-2500 if it delivers 4k60p. If it "only" retains 4k30p like the Lumix GH4, then I think they cannot upp the price beyond around US$1600.
To give you an idea what you need to get 4K video recording at a 60FPS framerate today, here are some examples. The Panasonic HC-X1000 4K costs US$2700, and has a rather small 1/2.3'' sensor.
Snoy's competition is called FDRAX1 4K, and costs US$4500. It also uses a small 1/2.3'' sensor.
The Canon C500 can record high framerate 4K video using a big sensor, however, it costs a staggering US$20.000.
The GH series was never intended to be a big seller, so don't expect to see low price on the unit when it comes. It is expected to sell only to the most video interested enthusiasts, and to some pro users.