What non-metal genres out there do you think most metalheads would actually get into? There are lots I can really think of, like psychedelic rock, garage rock, prog rock, southern rock, stoner rock, post-rock, grindcore, neofolk, dark ambient, classical, jazz fusion, free jazz, grunge, drone, anarcho punk, horror punk, crust punk, hardcore punk, D-beat, IDM, psytrance, metallic hardcore, martial industrial, power electronics, harsh noise, death industrial, EBM, power noise, industrial rock, gothic rock, deathrock, darkwave, neoclassical darkwave, underground/old school rap, psychobilly, outlaw/alternative country, noise rock, gabber, speedcore, breakcore, powerviolence, electro industrial, aggrotech, and shoegaze.

Depends on the metalhead.

That depends on the metalhead. If they are attracted to metal for its shocking lyrics I would suggest something like horror punk or industrial music. If they are interested in it for the technical aspects then classical or jazz.

Well a Metalhead would get into the more electronic stuff because it's related to Industrial Metal, the same way they would get into D-Beat or Punk because it's also related to Thrash (Thrash came from). I like Outlaw country because it has the same attitude that I can relate to while listening to Down or Pantera. But ultimately it does depend on the metal fan.