avant-funk?????

Naked City "derivatives" are:
Arto Lindsay and Ambitious Lovers;
Praxis "Transmutation" with Bill Laswel, Buckethead and Brain.
Company with Bill Laswel and other crazy guys may even seem to avant and heavy...
In Miles Davis you should search much later than Bitches Brew and try out "You're Under Arrest", "Amandla", "Decoy", "Tutu", "Do-Bop"...

You'd probably also like John Scofield's "A Go-Go" with
Medeski, Martin and Wood for something more recent.

Don Cherry: 'Brown Rice'
Joe McPhee: 'Nation Time'
Luther Thomas Human Arts Ensemble: 'Funky Donkey'
Sun Ra: 'Lanquidity'
Miles Davis: 'Black Magus', 'On the Corner', 'Get up With It'
Larry Young: 'Lawrence of Arabia'

..are a few that come to mind. A little more 'jazz' than funk for the most part, but good stuff. Joe McPhee and Luther Thomas somehow accomplished the feat of creating funky free-jazz, and there's a number of musicians/groups that are part of that particular scene who created some great stuff with the same kind of sound. A lot of the electric early '70's fusion that was part of the 'kozmigroov' sound/scene (and that includes the Herbie Hancock stuff from that era, as previously mentioned, as well as the Miles and Larry Young on my list) provide a very appropriate definition of a certain kind of 'avant-funk'. For detailed Kozmigroov information and recommendations, check out this link at Kozmigroov at FreeForm

Ronald Shannon Jackson and the Decoding Society W/Vernon Reid before Living Color.

Defunkt w/Joseph Bowie, Lester's brother

James Blood Ulmer

Ornette Coleman's Primetime Band

M'Base stuff

All the above are really good.

Meisterkleef,
Scofield's "A go-go" is rather classic funk than avant-funk that has its primary roots from New York underground world.