Keeping weight down will be important. More weight means more thrust needed, more thrust means more amp draw, and more amp draw means shorter battery life per X amount of battery capacity. Battery capacity is a double edge sword; if gives you more available power, but it is heavier and requires more lift to carry around.
600 grams of all up weight, with a 3300mah 3S battery (~250 grams), spinning 8x4.5" props on 1000KV motors could give upwards of 15 minutes of flight time with moderate maneuvering. At full throttle somewhere around 85 watts per motor (<8 amps) to extend battery consumption. Medium size props spinning relatively slowly limits your lift capacity, and is crucial to keep the overall weight down. The heavier you get from 600 grams could mean stepping up to 9x4.7" props for adequate lift and more current per motor and less flight time per charge.
Edit: It would be just about impossible to do a FPV copter less than 700 grams. Even when going to light weight 1400kv motors swinging 10x4.5" props, with a 3000mah 2S battery, on a Bat Bone body it is just a nudge over 600 grams. 15 minutes of flight time, but adding 300 grams of FPV gear and there isn't enough trust left for maneuverability and it drops flight time in half.
From experience, building a high power/trust setup to lift big heavy batteries to get longer flight times isn't a good solution either for the Rotor Bones components. They're just not up to handling 5.5Lbs of thrust and 3Lbs of all up weight; motor mounts break from torque, the extra weight tears up frame parts on crashes, and getting a good CG on the Bat Bone body is impossible without mounting the battery forward of the body itself. High power/trust setups need heavier duty frame parts which adds to the additional weight and where you start getting into multiple batteries to negate the additional power again from the additional weight for more air time; it is a never ending cycle.
In the world of automotive enthusiasts there is a saying: Fast, Reliable, and Inexpensive; pick two. It means to be fast and reliable will be very expensive, to be fast and inexpensive means it will not be reliable, and to be reliable and inexpensive it will not be fast. There is a trade off to be made. Weight, Power, and Flight Time; pick two.