CyBranding was suggested, but studying the site, I don't see a way to find hashtags from topics or terms - only from known tags. What's more, when I searched 'hashtagify' - http://hashtagify.me/#hashtagify I see 4 tweets in 4 days, and related tags: Twitter, Snowden, Prism, and Hashtags. Hashtags would be relevant. The rest...?
Cybranding: "no related tags" is what I get.
In the Cybranding http://www.cybranding.com/ - where to sign up / pay?
I would add to the list, and you are most welcome to, but, sorry, I don't see them as doing anything yet.
http://socialmention.com - studied it, but do a search, see the tags they suggest, judge the relevancy, and draw your own conclusions. Search the same thing months later, and your conclusions may be deeper. I wonder about how/if they update related tags, and what they do for an algorithm to rank tags for relevancy and reach...?