There's a rumour that Nick wil be back, at least briefly, at some point - thinking with my head and not my heart (which is still broken!) there are three big points in favour of this argument that I can see.
Firstly, that Dougie was very much around for most of the first three episodes, and could easily have filmed extra scenes for a later episode, which ITV are keeping very firmly secret so as to not spoil the surprise. Obviously, also still around is Lucy Brown - and looking like both Jenny and Claudia, so could have also filmed extra scenes as one or the other (or both!).
Secondly, the writers have known since the end of S2 that he was planning on leaving early in S3. I really can't believe that a suitable ending to the Nick/Claudia/Jenny storyline has not been written in at some point this series, even if only a brief conclusion. Also, I seem to remember an interview where it was confirmed that Claudia Brown would make a reappearance at some point......
Thirdly, the easiest way to persuade fans to adapt to the new characters is surely some closure of storylines with original characters! If we see Nick and Claudia reunited, or Helen finally meeting a suitable ending, how much easier to move on to a reinvented version of Primeval?
If these things don't come to pass, there will be an awful lot of disappointed fans, who will be less inclined to stick with Primeval and see how things develop, that's just human nature - we all like some kind of a proper conclusion, preferably a happy one, but some kind of closure for better or worse. Even Nick finding Claudia again in a parallel universe and getting killed alongside her would be better than that empty death at the hand of Helen the mad woman, surely!
If this is all wishful thinking, then I for one will be an extremely disappointed fan, as things could have been concluded so much better........