It seems to me that mirrorless cameras are here to stay. Canon and Nikon still haven't produced their own versions but I'm sure they will come up with their mirrorless cameras eventually. There are indeed persistent rumors of a Nikon mirrorless camera.
On that note I think it would be cool if Canon and Nikon made their mirrorless cameras compatible with existing lenses. That would leverage the strength of their existing lens lineups. Because mirrorless cameras have thinner bodies and the sensors are closer to the lens, compatibility would probably require an adaptor - would be nice if it preserved all functions such as autofocus, metering and vibration reduction.
Some may say that using traditional lenses on a mirrorless body would defeat the purpose of geting a mirrorless system (more compact size). I disagree - compatibility won't preclude the development of mirrorless lenses (presumably smaller and more compact). Owners with existing lenses though could use a mirrorless body (while keeping their existing camera) for more casual shooting or as a compact backup. With the ability to use regular lenses, mirrorless camera buyers also won't feel like they are taking a leap of faith on a system with potentially slower development of lenses.
Would be nice if they had electronic shutters too with unlimited sync speed.