Should I do another one for next year's Gfest? If so, should I elaborate on the story of the first (More about Miguju), or should I go a new avenue and use a different monster?
Go with a new opponent
yes, I think you should and going with a new story would be best.
I think a new story would be best. It would be fun to see Durontus face another challenge in the form of one of his other foes. those kinds of things works out so well in mini-comic form because their quick easy reads with lots of fun action that gets people more invested in your work. Plus its wonderful to diversify your portfolio with new work.