In that this game utilizes a great storytelling foundation to take the player through it, it would seem awkward to "jump into the past" and experience a new location and chapter after the fact - unless it was done using some form of a "flashback" as the set up. Anything else (other than a flashback situation) that sends us back in time seems like it would be a disconnection of what has up to now, been a seamless experience in a free roaming storyline.
Personally, I would love to have the expansions come out with both variances - one of them being a closed story flashback (having no effect on or allowing a return to the open world), while the other expands on the story beyond the point where we were left when we completed it and introduces some changes to the world we once explored. These would entail new creatures that have occupied the lairs of former beasts we slew, new gear and new quests/contracts to bring a level of evolution to the world we left. Even if they randomized some of the creatures, that would provide the degree of apparent change needed to convey a sense of time having passed.
Of course the second one has some design obstacles, such as administering to all the possible endings a player may have arrived at; but it appears that many of them could be more or less ignored (or simply implied by conversation) or could be done with some alterations of what we see populating old areas and the local guards, etc. The possible endings" could impact the expansion's setting and be what determines how we begin the expansion (and setting up the accompanying adventure hook), as well as affecting specific aspects of the forthcoming story itself (NPC's, encountering old friends, etc.) as we see how our previous results play out.
I believe that these guys have the skill to pull this off, but until we hear more we will have to simply wait. They have done a fantastic job so far, and I am sure that it is in their plans to impress and entertain us once again. Whatever they do, I will be eager to jump into it and spend any time I can experiencing the Witcher's world and its host of colorful and engaging characters.