Won't say it's not do-able, but it does sound very complex, in that (from what you describe) the computer will need to know about current local (to you) weather, current local traffic, current local terrain,and so on in order to do the predictions you describe.
I'd be happy with a MPG computer that can both tell me what I'm currently doing in simple terms (like the Scangauge), as well as record what I've done in detail, aggregating the local details after the fact if necessary, so that I can later (when I'm out of the car) view and compare against previous times I've taken the same route. Video capture would be nice too, so I can remind myself why I used brakes on a particular day and not on another (for instance).
Of course, I can sit here imagining I would take advantage of it, but probably wouldn't in actuality since there are other things I'd rather do instead... so the Scangauge is just perfect, in fact.
