Another method is to simply change the key of something you have already. Is anyone singing at this point? If you feel you are in some sort of rut, take something you have already and modulate it up a half step, a whole step, etc. This may help you in exploring new ideas and perhaps you will discover something unexpected as a result.

As stated above, most people gravitate towards a certain key simply because it is a comfortable range and yes; the guitar gravitates towards keys such as C, G, D, and E. Dropped D is also a great tool, and below is a tutorial I did on Dropped D styles that use metal. Let me know what you think;

http://guitartricks.com/tutorial.php?input=828