Flight time isn't all about battery, but also about weight and motor performance. Low-power motors and high capacity drone battery(you should choose high capacity with light weight capacity Lipo battery).

Batteries : The common battery technologies used in drones (LiPo, LiFePo) should not evolve much more than a 10% increase in energy density in the next years. Flight time should be improved in the future when new battery technologies will become more broadly available (i.e. less expensive and mass produced) and safe enough to fly. But now you can choose an high quality drone battery which has high capacity, light weight and small size.

Battery recommend: Tattu drone batteries;

Motor performance could be improved in the next few years but that improvement will not be very significant (probably less than 10% flight time increase). Choose a high performance with low-power is our standard.

Motor recommend: T-motor motors;

Weight : A lot can be done to reduce weight on drones, using different materials (complex composite materials or metal alloys). It can allow some weight reduction (but that weight reduction is highly dependant on the precise UAS type) and a subsequent flight time increase. But this will come at a huge price increase.

For example: Lumenier QAV250 Carbon Fiber Edition.

New ways of storing energy more efficiently can be applied to drones : fuel cells are currently being experimented by a lot of companies and that technology does represent a huge potential. However, fuel-cells do not deliver a lot of current right now, which limits their use to very lightweight drones.
You can read more form this link:
http://rcfpvplane.com/when-will-drones-have-longer-battery-life-and-longer-flight-time/

Flight time is directly proportional to the mah of the battery and inversely proportional to the weight of multirotor. More mah of battery will lead to increase in weight which also decreases the flight time. So you need to find a sweet spot in between this. These are main constrains related to flight time. Others are materials of frames, efficiency of motors etc. Nice post