To probe the sources of ultra-high energy cosmic-rays, next-generation ultra-high-energy detectors such as GRAND, BEACON, and the radio upgrade of AugerPrime, will target the detection of cosmic-ray, gamma-ray and neutrino showers with inclined arrival detections. These inclined showers develop higher in the atmosphere and over longer distances than vertical ones, which affects the radio emission in non-trivial ways. Using Monte-Carlo simulations, we evidence two major novel features of cosmic-ray inclined showers: a significant drop of more than one order of magnitude in the radiation energy and a new polarization pattern. We explain the former by a coherence loss in the radio emission of inclined air-showers, while the latter possibly indicates a synchrotron emission becoming relevant at large zenith angles. If confirmed, these two effects could have a significant impact on the detection and reconstruction strategies of next-generation radio experiments.