I'm about 3/5 done with the songwriting for my band's debut demo (http://www.myspace.com/manifestoblivion, all we have are rips of some dated Guitar Pro tabs so far). I have one instrumental/experimental sort of melodic metal track, another of a similar type but less experimental and more folk metal/neoclassical influenced, and I'm collaborating with the lead guitarist to write a new track that fuses mid-tempo groove metal with faster thrash/death (at different parts of the song).

The first two tracks came to me naturally over time, but the newer one is tricky. A few heavy or thrashy riffs have already come to me, but none of them are exceptional, and I'm in a block from writing new material of this style. Before, I'd listen to some similar bands and a new melody or idea would come to me naturally - but even after listening to Deicide, 80s Metallica, Slayer, Destruction, and Kreator for hours on end, nothing's coming to me.

If there are any thrash, death, or heavy/groove metal writers here, I'd appreciate some advice on coming up with new riff ideas. I'm mainly writing thrash and death metal riffs, but I also want to contribute some heavier riffs to the song.
i like the bands you mentioned but using them for inspiration with naturally make you sound so so because so many have emulated their styles,,,get your hands on some musiv by cynic,death (from human to sounds of perserverance) athiest,watchtower,theory in practice
these band will inspire you and probably make you think of something you hadnt all except for death havent really been emulated to much
i usually just pick up my guitar with the sole purpose of kicking its ass. thats where i get my inspiration from. a vicious 6 string beating set to 32nd notes