Don't expect a new engine at least until the next refresh (5 years or so). Well, hopefully by then we will have the Danki so this question would be moot. Nissan does not offer engine choices in any of their models (except Titan and Altima i believe) and I don't believe Cube will be an exception. The 1.8L should be powerful enough to zip you through town yet get decent milage. I would hate to see them go the xB route and say drop the Rogue/Altima 2.5L, 170hp unit in there.
Side note: Cubes MPG is only +1mpg to Altima 2.5 and Rogue with an engine .7L smaller and 50hp less output. Whats the deal?