To probe the sources of ultra-high-energy cosmic-rays, the next generation of radio experiments such as GRAND, BEACON and AugerPrime Radio are now focusing on the detection of air- showers arriving with very inclined trajectories. These inclined showers have a large footprint that allows us to instrument large surfaces at low costs, but their radio emission exhibits numerous features that differentiate them from vertical ones. Using Monte-Carlo simulations, we evidence here two major novel features of cosmic-ray inclined showers: a significant drop of more than one order of magnitude in the geomagnetic emission amplitude 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. These two effects depend both on the magnetic field amplitude and are challenging to describe with the existing interpretations of the radio emission but could affect the detection and reconstruction strategies of next generation experiments.