Two things come to mind:
The fairly obvious: convince MTV to air the video for "Madhouse" in 1985. It's been enduringly popular, and the original stated reasons for MTV refusing to air it (insensitive to the mentally ill) make no sense. Given the positive relationship between Scott Ian and MTV (I'm thinking of the "Let Anthrax Destroy My House" promo on MTV in the late '80s), and given Ian's TV-friendly personality -- as demonstrated by his numerous appearances on VH1s "Best Of" specials -- it's easy to imagine "Madhouse" becoming a sleeper hit in '85.
The commercial one that will probably make you cringe: put a power ballad on 1987's Among the Living. Don't give me that look! Even if you don't like power ballads -- and seriously, if you don't, why not?? -- they were essentially the key to metal acts becoming successful mainstream acts in the 80s.
Put a song like "November Rain" on Among the Living, and you've got a band with the potential of... well, Guns 'n Roses.