A jazz/fusion guitarist who I think metal musicians should be aware of is John Scofield. He comes out of the post-post-bop school in the 1970’s and 1980’s, alongside players like John McLaughlin, who grew up listening to several generations and evolutions of jazz, and in their time, took the art form to new places, infusing it with funk, prog, rock, and more. Scofield worked as a solo artist and with a number of A-level musicians, like Miles Davis, George Duke, Herbie Hancock, Joe Henderson, Billy Cobham, and dozens more.
The reason why he is relevant for metal players is that he is out there. The guy is a brain. If you like metal, and frequent this website, then you probably like progressive music. Well Scofield is like a prog guy, except with formal jazz training and serious chops acting as his musical foundation. So when he goes out, he is doing it from a place of understanding what it means to go out. His lines are crazy, but they are also often very beautiful.
So you may find the below video, a masterclass from 1983, to be as useful as I did. Scofield runs down the major and melodic minor modes, and also reveals a few pentatonic tricks and general thoughts on improvisation. He goes through a lot of this stuff pretty fast (imagine taking notes in this room), but the glory of video content is that you can hit pause and rewind!
Keep in mind some of this stuff is very complex, and some of it may be new to a few of you. Don’t let that deter you. Take from it what moves you. There’s no one way to learn theory, as long as you are dedicated and excited by the ideas!