With a planned release date for this September with a pricetag of ~$70, the G500 will have a resolution range from 200 to 5700dpi [700 more than the G9x, 100 more than the Mamba]. Logitech claims that the G500 can process sensor data at a rate of 12
megapixels per second, and can detect movement as fast as 165 inches
per second (as if your twitch skills are that good).
Its grip will have a broader thumb rest and smoother pinky/ring finger ledge [similar to mx1000/Revolution/1100] and its scroll wheel get the hyper-fast/precision toggle upgrade that G9 users are familiar with. DPI adjustment buttons have been relocated to the outer edge of the left mouse button while the scroll wheel adjustment button takes their place below the wheel. On the side we have our usual forward/back keys with and added middle button. All buttons lose their rounded design for the G9's squarer look.
Available weights are fewer than current G5 users have available to them. The G500 will still have the round 1.7 and 4.5g weights but with 2 fewer slots in the weight cartridge, allowing up to 27g [G9 allows 28g, G5 allows up to 36g]. This affects our resident King Admin, Album, as he prefers his mice to be heavy as a brick.
Personally, I'm really excited as these were the exact features, new G9 style scroll wheel + extra side button + smoother grip and thumb rest, I wanted when I was mouse hunting earlier this year and I will be saving up for one.