Though it has many uses, such as general navigation, Waze was created largely for commuters to fight traffic, yes? Then why doesn't Waze have traffic-based alerts for planned routes?
The basic idea is that Waze would use a combination of current traffic info and past traffic patterns to determine when a user has to get on the road to get to a destination by a certain time.
Implementation 1: Get there on time!
A user tells Waze that they must get to a destination by a certain time. (Work at 9:30 AM, for example) Waze constantly analyzes the route's traffic patterns, and sends the user an alert a few minutes before they must leave.
Implementation 2: Spend less time in traffic!
Perhaps a user has some flexibility as to when they can get on the road, but they want to spend as little time in traffic as possible. (Leaving work between 6:30 and 7:30 PM, for example) In this implementation, a user can enter their destination and a window of time they'd like to leave or arrive: Waze then tells them the best time to leave, based on past and current traffic.
Given the wealth of current/past traffic data Waze now has, the ability for users to plan routes ahead of time using Waze is not only possible, but likely one of the most useful features Waze could have.