It's sort of a combination of things...just playing around, really & looking at flavor menus from websites...gets me thinking...lol. One website that has some great flavor/filling combinations is www.thegrandfinale.com
Sweet Suggestions is what she calls her flavors list. Plus, I just wonder if something will work and try it, sometimes. Like tonight...I had some leftover caramel (could've just eaten it all by itself
) but I started wondering if it could just be mixed into some buttercream. Well, I wanted to try making IMBC (Italian Meringue Buttercream) so I made some and then I warmed the caramel in the microwave (it had been in the fridge and was super thick) then let it come to room temp and then I added it to the IMBC, along with a little cocoa powder - made it a sort of mocha-caramel flavor...really yummy! When it was done, the texture of the IMBC resembled more of a regular BC.
Just start thinking of flavors that you like, and then think which of those flavors would go well together...make a list if it helps you to see them on paper & keep it straight in your head. The next experiment I'll do, in a few days, is a chocolate-orange-almond chunk cookie, maybe pecans instead of almonds...we'll see!
Don't be afraid to play around...just write stuff down as you go in case it turns out to be great!