The placement of will at the end of the utterance has effect of stressing it, so any context where the speaker has a special reason to create such an effect.
One such reason could be for contrast:
A: I'm disappointed things haven't gotten any better.
B: They soon will.
Speaker B wishes to make a contrast between past and future, and so stresses the word will by placing it at the end of the sentence. This emphasises the meaning of the word will, an emphasis which in this case has the purpose of reassuring speaker B.
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