One band that has to be mentioned is Fear of God. 13 years after its release, their album "Within The Veil" is still one of my top 2 favourite metal albums of all times, especially because of Dawn Crosby´s (R.I.P.) vocals. Their second (and last) album "Toxic Voodoo" (released in 1994) didn´t impress me that much. Dawn´s vocals somehow lack the depth and intensity they had on "Within The Veil" and the music is less original (possibly also due to the fact that the entire line-up except for Dawn had changed), more heading in the death metal direction. For further reading I recommend to visit the official website
www.fear-of-god.com. Unfortunately, "Within The Veil" isn´t available anymore, but the webmaster of the aforementioned website might have some used copies for sale.
Another band which comes to my mind is Sieges Even. On their first album "Life Cycle" (released in 1988) they sounded a lot like Watchtower, but on their follow-ups "Steps" (1990), "A Sense Of Change" (1992) and "Sophisticated" (1995), which all sound different from each other, they had developed a style of their own. "Steps" is possibly their most technical album, whereas "A Sense Of Change" is much more melodic (hardly to be considered metal anymore) and "Sophisticated" some kind of mixture of both. Now they are finally back again and I´m already curious what they will come up with this time.
One of the most original thrash metal bands are definitely Mekong Delta, who had a really unique and also somewhat weird sound. Like Coroner and Anacrusis, they already had their first releases in the eighties ("Mekong Delta", "The Music Of Erich Zann" and "The Principle Of Doubt"). Their album "Dances Of Death" (1990) probably contains one of the longest thrash metal songs (about 20 minutes), which was composed in the form of a rondo. On nearly each album, they also covered a classical piece, e.g. Mussorgsky´s "Night On A Bare Mountain" on "Dances Of Death". But maybe you already read all this in another thread. I recently heard that they might release a new album this year.