It would be cool if there were a hierarchy of hashtags. Say the hashtags #HaloReach, #HaloCE, #Halo3ODST, etc were all children of a generic #Halo hashtag. The idea would be that when I filter on the #Halo hashtag, child hashtags would automatically be included. Although for simplicity, rather than change the filtering mechanism to deal with a hierarchy, you could, at thread create time, append the parent node of each hashtag to the thread. So if I create a thread with #HaloReach, #Halo is automatically added. I think it would help make the whole hashtag mechanism more useful.