I can't knowledgeably advise you as to what is happening, or is going to happen, but -several- Toyota people have publicly stated that the "new Prius" will be smaller than the current car. If this is in fact what is introduced, they will probably continue with the current car, having two Prius models. They -might- add plugin capability to the current car and reintroduce it in a few years as well (with minor body tweeks).
This also meshes with Toyota's floating of the concept of a "Prius line" of cars. Several models, all called "Prius", but one a "Prius compact", one a "Prius midsize", perhaps a "Prius mini-SUV", the current midsize SUV (Highlander) called "Prius SUV", etc.
If that is what happens, and you want a car that is the size of the current car, then waiting will be superfluous.

Think of all the fuel you could have saved.
The hazards of trying to predict the future.
