This is just speculation though - Intel haven't confirmed or denied mainstream moving away from quad-core. Bear in mind the mainstream chips are just "full fat" mobile i7s anyway; there's been absolutely zilch in terms of rumours of mobile moving beyond quad-core. (Aside that is from the old Clevo machines which had a 6-core Ivy Bridge-E CPU).Cannonlake will be the first set of consumer cpu's with a 6+ core top end cpu.
More speculation for you - Cannonlake is meant to be a die-shrink of Kaby Lake, meaning the same socket type. The socket is set up for quad-core and thus Cannonlake itself is likely to be quad-core.
IMO the best chance of anything greater than 4 cores mainstream is with whatever comes after Cannonlake (Ice Lake according to Wiki). The other thing that could give a clue will be when 6-core Enthusiast chips are phased out...