Struggling with adventure prep

I recently started a superhero campaign with Fudge, and we've got two really shaky sessions under our belt. Thanks to busy summer schedules, I've had weeks to prepare for this session and I've been struggling with it. Mostly I've put it off because I've had no inspiration and it's not been doing a very good job of brewing subconsciously. (Usually some times not intentionally thinking about a project produces something... but it's not helping this time.)

One of the things that causes me to struggle with a campaign and preparing adventures is not having an overarching theme of some kind. And if I don't already have an idea when the campaign starts, it's often difficult to apply an arbitrary theme. Characters with well-defined motivations and goals help a lot, but I'm faced with a lot of "blank canvas" issues on this one... "we're a superhero group, we fight crime" leaves me so many options that I have trouble deciding on a direction. This happens mostly because I'm wanting to tie plot elements into the characters' backgrounds, motivations and goals. I don't have a great deal of that to work with with this batch of characters. Everyone's pressed for time and there hasn't been a great deal of work put into any of the characters. (Or the campaign, so I can't blame my players too much.)

I've started the first adventure with the only strong plot hook I could find in the three characters, and after many weeks of brain-storming, I've come up with a theme for at least a multi-adventure sub-plot. It may become the main plot with unrelated adventures woven in between, but it'll work well as a sub-plot if I find something else to take the main slot. I've an ideas for a stronger main plot, and I've got the character hooks to keep it from being totally arbitrary, but I'm not sure if I want to go in that direction or not yet.