I have a GH2, D800 and 60d. So far the GH4 looks like a very sensible next step, but typically I have 2 big jobs that need shooting before it'll be out.
I have a speedbooster, and a growing collection of nikon glass. I do a lot of wide lens stuff currently with the tokina 11-16, but I miss the canon 10-22, bigger range and less barrel distortion. I shoot a bit of timelapse now and then
Theres only about $250 separates g6, d5300, and gh3. Don't especially shoot in low light, but have found the gh2 blocky noise and fizz in the shadows to be an issue. If the G6 is no different in that regard I'll rule it out. (Some people say its better, some dont). I like the dynamic range of the d800 video, but thats about it.
Really need a decent fast ultrawide for M43 or APSC too. (12mm doesn't cut it on M43, I need 7-9mm or 10-12 on apsc).