I wouldn't be able to throw together a full list for avant-garde metal, my only exploration of the genre is primarily through John Zorn's many projects.
Of Zorn's projects I would certainly nominate
Torture Garden as fair game, there are some very metal moments weaved into most of the album, maybe John Zorn's recent album
Psychomagia, Moonchild's
Songs Without Words, three pretty excellent albums.
Outside of Zorn's convoluted world maybe Ministry's
Psalm 69: The Way to Succeed and the Way to Suck Eggs. Although it seems rather tame in light of Zorn's work I'd say it's experimental enough to make the list if Celtic Frost is going to be there too. I'm a huge fan of Rekombinacja's
Domestic Props, which may be unreasonably obscure next to the rest of the albums on the final list, but it's an amazing technical feat I'd love to see included.
If you're interested in hearing it:
REKOMBINACJA - Domestic Props - NKS prod 114 [experimental metal breakcore cybergrind glitch noise from Poland/Wroclaw]
Otherwise I think that's pretty much all I've got to add in terms of experimental metal.
EDIT: Would be nice to see Gridlink's
Longhena too.