I've hit writers block a bunch of times, I find the more you write the easier it gets, so if you're stuck then start writing crap and see if it leads somewhere (just write the scripts in Word if you don't want to publish them or start a non-donor account to play around with). Even crap can be re-written into something good later when you're inspired, so it won't be a waste.
I'd also go for an every day concept that's got a lot of flexibility so it's harder to write yourself into a corner - such as a workplace setting (office, news readers, bookstore, etc...) rather than anything too specific like aliens on a spaceship or fish in the ocean.
Another trick is to "steal" existing characters from tv shows or movies and write a script using them (it's basically just a fan script, but you can mix and match characters from different shows and put them in a different setting). Later go back and change the names, catch-phrases, etc... so that they're unrecognisable.
This is a really handy way to get yourself going again as a lot of characters will write themselves - you know how they would behave in any situation and you just put it down on paper.
An example might be to take Bender from Futurama, Quagmire from Family Guy, Sparks from Sealab and Tim from Home Improvement. Pretend they're all mechanics working in a garage, write the script on that basis, and later change their names to Jim, Steve, Ted and Dave, then make them all human.
That's how Melvin came to exist - he's how I imagined Shake from Aqua Teen Hunger Force would look if he was an office worker.
You might want to step outside the limitations of Stripcreator as well and just write without having to worry about the limited character/background selection, only being able to put 2 characters in a frame at once, etc... and just write a script as if it was for a cartoon. You can adapt it later easily enough (or try a different format)