Via the possible aid of Celtic migrations displacing any potential non-Celtic groups as well as the Helvetii quickly becoming aware of Galba's death during the Year of the Four Emperors (thereby avoiding the massive punitive campaign against them). What if Continental Celtic (either Noric, Lepontic, etc) managed to survive roughly in the areas of Switzerland, Liechtenstein, Voralberg and Tyrol with scope for a small state later down the line?