Well, "ambitious" might mean in scope, and not in size. We will see I guess. Personally, I'd probably feel better with a tight 40-50 hour gameplay per character as well (Similar to the length and style of Witcher 2). But you never know, maybe they will do something unprecedented, that will not burn you out so much even after a 100-hour playthrough.
Although I must say that I did 2 playthroughs for Witcher 3 so far, spending around 300-400 hours, and did not burn out quite so much. But of course if they are making it big, they should avoid that "spoils of war" kind of stuff, like I said before. Or maybe turn those stuff into interesting little story bits or something. Aside from those I did not feel like there were that much of filler-content in Witcher 3 (compared to other large or open-world RPGs, I'm looking at you DA:I), but of course it can and should be improved on, now that they have a significantly larger budget, IF they are planning on making in at least as big of a game.