mythsofmetal

Try these couple if you don't know them yet. Though, Thy Catafalque isn't very obscure, I feel it's worth a chance in case you happen to not know them.

Thy Catafalque -- Szervetlen | Thy Catafalque

Mourning Mist -- Rise And Decay | Forever Plagued Records


To be honest, though, I feel like even really avant-garde metal bands can have their style (or styles of albums/songs if the stuff differs within their discographies enough) categorized accurately with enough adjectives, within reason. Also, were you asking more for bands with a unique and interesting mix of styles, or more-so a band which has a lot of variety between their own songs/within their own discography?

MicShazam

I guess any kind of interesting stylistic mix is good. I mentioned Stam1na for instance; their albums sound kinda samey, but they also sound very idiosyncratic, like some kind of melodic, prog thrash thing with an odd melodic sensibility.
Bands that try out all kinds of styles between songs/albums are fine too, unless the individual songs become too genre-typical. Really just anything that you feel avoids (easy) categorization or pigeon-holing.

About your two recommendations: I do know Thy Catafalque already as my brother is into them, but I need to check up on them again. It's been a long time!

I don't think I've heard Mourning Mist so I'll check them out too and see what I think.