As the owner/op of a couple of C300 II's, I can say that the C500 II seems like a significant upgrade at a justifiable price. 4k60p, full frame for when requested, internal RAW, the more adaptable form factor, internal stabilization (if isn't useless). If they also manage to increase the DR, handle shadows better, and provide better implementation of NR then I think it is well worth it for commercial work. I also don't quite understand the complaint about stock color science. The C300 II definitely had it's look, but it also had a relatively large number of stock color profiles to choose from. It wasn't like you were stuck choosing s-log 3 and a general white balance like on the Sony cameras.
You like C300 II color, Eric's not a fan (compared to ARRI), and I did my best to match the C300 II to optimized color I did for the 1DX II (1DX II still better IMO), supporting evidence: http://brightland.com/w/downloads/filmic-skin-for-canon-c300-ii/ , http://brightland.com/w/downloads/filmic-skin-pictures-styles-for-canon-dslrs/ . It's one of the reasons I sold the C300 II. I did have a chance to try one of my optimized color settings on the C700 FF, and it looked pretty good, however was never given permission to share the results. Alexa & Venice look better than the C300 II (by quite a bit, depending on the shot in real world light), so hopefully Canon updated the C500 II to be more competitive straight out of camera.
Agree with other comments about price (I purchased the C300 II at launch for $16000. After realizing it was really a 4096*.66 = 2.7K (at best) real resolution camera with terrible aliasing on fine fabric/patterns, I felt ripped off). Kudos to Canon for finally providing honest/true 4K with a 5.9K sensor, however IMO it's an $8K camera relative to the current market.