Hanlon's Razor: "Never attribute to malice what can be explained by stupidity." We've already seen that Fenris has very petty reasons to want the world destroyed-- namely that he wants to tear everything down just because he wants to murder everybody. His traditional association with Ragnarok implies a relationship with making something new in the place of what is old, and we see that idea in Thor's view of Fenris as a god who easily gets bored with what's happening now and wants to make something new and interesting. Goblinoids are less his "pet project" and more like that New Year's resolution you try out every January but forget about by the end of February. He has completely understandable motivations, so far as "ooh, shiny!" can be considered a motivation, so there's really no need for a "master plan."