I've recently came up with a great riff and now I want to add on to it and write a guitar instrumental. If you want to hear the riff, listen to it on my profile. It's the one called Thrash Riff1.

I have a few ideas of verse riffs, but some aren't so smooth and some aren't powerfull enough for what I want (thrash).

I just want to know if there is a technique for writing verse riffs or coming up with ideas for verse riffs. I'm not asking you guys to help me express myself, that would be stupid. I just would like help on how to come up with verse riffs.
Look at the key signature, for a start. That's all the advice I really have, sorry.
just mess about with your lead playing over the top of it

eventually you'll get something good
'The Blues is Easy to Play, but Hard to Feel'
Jimi Hendrix
Anything goes when writing instrumentals. For some great ideas of what directions you can take, listen to Buckethead (he does have a grindcore/death metal-influenced album, The Cuckoo Clocks from Hell, but almost all of his stuff is wroth a listen).

One mistake that I commonly made in the past, though, was that I was too afraid of being repetitive, and I rushed ahead to the next riff all the time, which ruined the cohesion of the song.

Also, the above advice to look at the time signature is all wrong. Time signature changes can be fantastic if executed right. As I said, anything goes.