If you are asking about the specific phrase 'suggest otherwise', I think that (a) it's a little idiomatic, and (b) that 'otherwise' is a shorter version of 'something else'. I'm not sure - perhaps there is a stricter grammatical explanation?

Thank you both!

I think Gary is right that this is just idiomatic. I can't think of any other adverb that would make sense after "suggest."

Your question is a little confusing. Are you asking about the specific phrase 'suggest otherwise'?