In terms of either thrust or delta v, what is the next big thing?
There is a bunch of incremental research on adapting different ion thrusters and other pre-existing propulsion systems to cubesats. There are some experimental propulsions (especially if you count braking: deorbiting satellites as a kind of propulsion) - electrodynamic, or utilizing trace atmospheric drag - that are tested with aid of cubesats, although not intended as actual practical propulsion for nanosatellites - as normally propulsion is means to get a satellite into position where it performs its actual tasks - scientific or otherwise - these propulsion systems cost too much and leave too little room for equipment for these tasks. Cubesat becomes a test stand, not the actual target "customer" vessel.
The one currently being developed, completely novel propulsion system for cubesats, while interesting, is hardly a breakthrough: Electric Solid Propellant propulsion.
The promise of ESP is bringing fully functional, inexpensive and safe RCS capability - and even inexpensive low-performance main propulsion of performance much higher than typical inexpensive alternatives (cold gas, steam), and cost way lower than high-performance alternatives (ion). On top of moderate cost, the solution is really tiny - a single thruster weighing around 6 gram, leaving a plenty of room and mass for the cubesat's primary functions.
The obvious disadvantage, making it not-breakthrough, is the poor performance: the single RCS thruster can provide delta-V of order of 10cm/s to your typical 1U cubesat, and all ESP thrusters have a very modest 200-230s of specific impulse. That's better than cold gas or the likes, but worse than about all chemical (solid, mono- and bi-propellant), never mind ion propulsions. Still, the material safety meets the stringent requirements for cubesats, and the device simplicity vastly reduces its dry mass comparing to "competitors".
Of course RCS is not a highly-sought feature for cubesats, and the puny delta-V is making it hard to to treat as anything but a gimmick.