I'd argue that massive, world changing decisions shouldn't even be a part of the plan if they will end up being scrapped as it breaks immersion. If the developer knows it is unrealistic for the sequel to take into account many of the choices made in the previous game, then the choices you make in the previous game shouldn't be so significant so that if they end up being ignored or minimalized, it won't be as big of a deal from the player's perspective.
Take Mass Effect 3 for example. Saving the Human Reaper's corpse (and thus your relationship with Cerberus) and saving the Rachni (an entire race) are something one would expect to be very world changing things from the previous games. In the end, though, they are trivialized beyond belief, turned into a meaningless statistic in the former's case and pretty much swept under the rug in the case of the latter. I'm sure there are other offenders in that series, but I haven't played the series in several years so I don't quite remember all the choices.
But in the case of the Witcher 2, things like Henselt, Stennis, and Saskia's fates shouldn't be made unimportant - these are rulers of countries, or people with significant enough influence to be comparable to one. Siding with Roche or Iorveth is also a pretty big deal considering the allies you could have (or couldn't have) made. Even the fates of less politically important characters like Letho and Sile are also potentially important.
I know its a lot to expect, but at least when we're talking about the first factor I listed (the fates of the leaders of countries), those are things that cannot be trivialized without making me raise an eyebrow.