I think the problem with an external mic, is if you're parked on or near a busy road or footpath, then the cameras always going to be triggered and on, plus even if you record sound only, it still takes storage capacity to record constant audio.
Personally, I think the immediate future for dashcams lies with getting the basics right - lets get away from cheap lenses, low bit rates, fancy features that are unnecessary. Instead get the quality up so that number plates are readable at reasonable range and the picture is sharp and clear at all times, (maybe usable for non dashcam purposes as well), then worry about adding bells and whistles. For me dashcams have run before they can properly walk, or we've put the cart before the horse (if you prefer that saying).
I still wonder as well, having considered the fact a few people are running multiple cameras to get the best of both focal lengths, if there isn't some position at the high end for a dual lens camera - 1 fixed wide, and another with longer fixed focal length. You could even have Karcam's idea of a modular design where you could have 2 separate cameras with a common shell design, where one piggy backs and attaches to the base of the other so they stack one above the other (heat dissipation allowing).
Another radical solution would be to offer a camera with an M12 thread which enables the removal and changing of lens to any with an M12, thus opening the possibility of supplying a cam at a low price with a "standard" quality lens, but leaving it open for the end user to screw in another lens of higher quality or differing focal length. There could even be a cheaper lens free option where the cam is supplied minus a lens.
Another radical solution would be a camera with a remote lens mounted eg in the front grill or back edge of the bonnet to overcome the reflection from the windscreen. There are plenty of options to explore that potentially offer real picture gains going forwards, that offer more usefulness, than the latest 7 gps spell correcting self texting social media essay writing alert camera feature....that sells cameras but no-one really needs or even uses.
My philosophy is simple, let the pictures do the talking!