Oh man, is that a loaded question. I know this seems like a cop-out, but there are all kinds of great things to put in a fattie. My suggetion is to go to the fattie area and just start looking. Every once in a while you'll run across something that somebody did that they didn't like. But I'd saythat 99% of what I see on here, the chef says he/she liked it.
Other than that my only suggestion would be to think to yourself, "what sounds like it would taste good with sausage?" then go from there. And it doesn't have to be sausage either. That's just the more traditonal method.
The pizza fattie is a pretty good one that a lot of people try firsthttp://www.smokingmeatforums.com/for...ad.php?t=74952
Then there's more breakfast style like this pancake and apple versionhttp://www.smokingmeatforums.com/for...ad.php?t=75654
or the egg, cheese and chorizo fattiehttp://www.smokingmeatforums.com/for...ad.php?t=75932
that one has a nice rolling tutorial to help get you started
Then here is one using Italian sausagehttp://www.smokingmeatforums.com/for...ad.php?t=75655
Welcome to the SMF. Lots of friendly helpful people on here. Just ask a question and wait a few minutes and surely someone will be right along to help you.