What gives me pause about ne representing the conjunction here, is that the conjunction ne means "lest", or if you will, "that not". Tacitus' statement would seem to make somewhat less sense to me with these meanings inserted: The man cause for fear was, lest (or: that not) Germanicus, with the power of so many legions, much help from allies, and the Goodwill of the people, would have (read: take) power (now) rather than wait for it. This seems to be saying, with the negation inherent in the conjunction ne, that the fear was that Germanicus would not seize power, but I think that Tacitus was saying the opposite.Why? I think your original thought was correct as I mentioned above.
I do also have a bit of reservation about ne as interjection in this case: if it is, it seems to me that ut should be present as well, to render the meaning "...indeed that Germanicus...", but in this I may be trying to impose typical English syntactic construction onto a Latin sentence. I am not yet entirely facile with the use of the Latin conjunctions...
We probably need someone with greater facility to help us with this, such as @Pacifica or @Cinefactus.