There are a number of potential options for high-performance engines in the future: nuclear pulse propulsion, NTR, gas-core NTR, nuclear electrothermal, and potentially even fusion rockets.
However, most of these seem to be main drive engines that do not scale down much.
While there appears to be a movement towards cryogenic RCS rather than hypergol, this doesn't provide much improvement.
What options are viable for RCS thrusters with higher Isp than chemical allows?
(To be clear: this is about the higher-thrust-range thrusters typically used in manned spacecraft, spacecraft that dock, and landers. Microthrust electrical thrusters need not apply.)