Apart from some other bands already mentioned (Toxik, Détente, Znowhite, Coroner), the first one to come to my mind would be Hexenhaus and their excellent debut "A Tribute To Insanity" - definitely original, just listen to their 10-minute piece "As Darkness Falls" -, but I guess you´re already familiar with them. (Or is "Dejavoodoo" - I really like it very much, so thanks again! - their only album you´ve got?)
Another original band were definitely Hades, though I don´t know if they qualify as a "proper" thrash metal band. I mean, when I think about their debut "Resisting Success", I don´t doubt songs like "The Leaders?", "On To Iliad" and "Legal Tender" are thrash, but there are also some mid-tempo songs and "The Cross" might even be called a ballad. (Maybe this is a bit off-topic, but the last song on this album - "Masque Of The Red Death" - has some really cool bass parts!)
Mekong Delta also had an absolutely unique sound, but I think you´ll already be familiar with them.
The first three Voivod albums (I don´t know if "Dimension Hatröss" still is considered thrash metal) were also definitely original thrash metal albums, but I guess they count neither as forgotten nor as overlooked.
A probably quite unknown thrash metal band might be Erosion from Germany, who released their debut "Mortal Agony" in 1988. Maybe not as unique as Mekong Delta or Voivod, but not a clone of one of the big three you mentioned, either.
Definitely overlooked and forgotten are Original Sin and their only album "Sin Will Find You Out". I couldn´t say they had a unique sound, but they were definitely original insofar as they were one of the very few all-female thrash metal bands to officially release an album! (And one thing is for sure: they can´t be a Testament-wannabe, because their album was released about one year before "The Legacy"!
BTW, I think the music is also worth listening to, though the lyrics are a bit too clichéd for my taste.)
Finally, one could also say that The Great Kat was original in a certain way
grin: ), at least on her debut "Worship Me Or Die!" (I think she´s still around), which features some of the most hilarious - or should I better say: crazy - lyrics ever. I mean, there are not that many bands that come up with choruses like "WORSHIP ME OR DIE! DIE! DIE! DIE! DIE! WORSHIP ME OR DIE! DIE! DIE! DIE! DIE!" (in - you guessed it - "Whorship Me Or Die!"), "SATAN SAYS, FOLLOW ME! SATAN SAYS, SHOUT LOUD! SATAN SAYS, FOLLOW ME! SATAN SAYS, GO TO HELL!" ("Satan Says") or "KILL THE MUTHER! KILL THE MUTHER! KILL THE FUCKER! KILL THE FUCKER! KILL THE MUTHER! KILL THE MUTHER! KILL THE MUTHER MUTHER FUCKER!" ("Kill The Muthers") - yes, it´s all printed in capital letters on the inner sleeve of the album. As for the music, she herself calls it HYPERSPEED METAL and most of the songs last just between 2 and 2 1/2 minutes. Actually, I´ve listened to this album just a few times, after I´d bought it (about 17 years ago). I mean, she definitely can play very fast, but I really wasn´t too impressed with the songs and the music does get a bit repetitive.
