I want to give a suggestion to create characters, do you like movies about spies? Might such time creating a spy agency (60 new characters, 4 could be a family), entrusted to protect and save Happy Tree Town of threats that could end it all. Sniffles could be the agency's scientist.
and too you can created a evil polar bear (based on Herbert P. Bear from Club Penguin) and his league of villains as the agency's ach-nemesis.
You can create a nephew-and-uncle duo, or a niece-and-aunt duo or a niece-and-uncle duo. They may make their debut in a episode based on an Argentine animated film named Mil Intentos y un Invento (in english A Thousand Attempts and One Invention)
For watch the film click it to these two links of YouTube (there is a video of this film in two parts)