This is an old question that was bumped to the homepage by the Community.
To be honest, I have never read or heard ～兼ねない used in a sentence that does not indicate something negative from the speaker's point-of-view. However, after reading the sentence in question and doing some research (and being a fan of Attack on Titan, if that helps), I found out that ～兼ねない simply means something that could happen, something that is not unlikely to happen, or something that is capable of being done, regardless of a negative connotation. Consider the following definitions:
quite capable of ...; might happen; not unlikely to
Next, let us see how its opposite is defined to gain more information:
〈ためらう〉 hesitate to do; be unwilling [reluctant] to do
〈できない〉 cannot do; cannot [【形式ばった表現】 can ill] afford to do; be not in a
position to do; be unable to do
From the above, I think it is clear that neither ～兼ねない nor ～兼ねる needs a negative connotation to be grammatical.
Now let us go back to the sentence in question. I can't post images right now, but checking the internet it looks like this sentence was spoken by Hanji, one of the good guys and a superior officer of Eren, the protagonist. Below are the original sentences and the ones before and after it.
Sources: 1, 2
To conclude, Hanji is saying that if it is really possible that
Eren (could control the Titans),
then the current situation that mankind is in could be reversed. She is not saying at all that this possibility is something that is negative from her point-of-view, as she is one of the good guys who are trying to protect mankind. In fact, from the sentences before and after, we can see that it is something positive for her
as she is looking forward to testing Eren's (other) potential power.