It doesn't seem plausible to me that Canon would release a 24mp APS-C sensor. According to Canon themselves, their BEST L-series lenses are only capable of resolving about 45mp of detail in a Full-Frame image circle. The current generation of Canon's 18mp APS-C sensors have exactly the same density as a 46.7mp full frame sensor, which would mean that a 24mp APS-C sensor would far outresolve any lens Canon has on the market right now, and probably for several years to come at least.
I think it would be far more beneficial to Canon to work on the IQ of their current 18mp APS-C sensors than to try and boost megapixels any farther. The only area where more megapixels makes sense is in larger sensors, and given they have dropped APS-H (or seemingly so), that means full-frame sensors.