Cool thread! I've been playing classical piano for 10 years now and have done theory for the same length of time.
I tend to choose my key (my favourites being C# or G# minor harmonic), then just jam on my piano around that key. When I find something cool, I play it over and over again, and try to put some left hand to it, which is usually my main problem, however in metal keyboards, left hand is not always necessary.
Another way I like it to play a part of a song you know well over and over, then as I play it more, I just naturally change it, then it's usable and my own work. An example of this is a Finntroll keyboard bit I played, then changed it and it (my edited version) ended up as a fill in my first song.
Hope I've helped!