A weather report for the planed route would be very useful. Especially for the longer routes. One may leave in sunny weather and find a snowstorm a few hundred kilometres away.

Allow edits by the author. This was a bit sloppy, but hopefully it's a good idea.

Is it possible to have a forward looking route-time based on confidence intervals from previously collected data. The idea being that if you're at your at your office and pull up your route you can see that if you leave in an hour instead you'll save 40min off your commute. It would also be helpful for planning purposes, say you need to drive somewhere tomorrow and be there at 7:30am, but aren't familiar with the route. You could graph an upper and a lower delay based on some reasonable confidence interval along the y-axis and time along the x-axis. It would be fantastic to be able to scroll along this "worm like graph" and get a traffic prediction just like you get a weather prediction. There could be a line going backwards where the data is known with labels at spikes for example Sea Fair and Bloomsday in Seattle and Spokane respectively. The past data would be incredibly easy to interpret if the Waze community could label it and would allow for better traffic planning by authorities. The data could be made available on your terms of course :) to authorities so that they could use something other than a phone for their own analysis and route planning.
Maybe this already exists. I rarely use Waze because I live in a small town and don't drive through traffic, but Waze saved my booty today when I saw a giant pileup and exited the interstate.

Incorporate weather (actual rain / precip vs. just clouds) as an overlay option of the planned route. If your planned route has rain over it, you'll know before you get there and can close the top or if on a motorcycle, get out the rain gear.