That -6 ability will be partially good as green tends to suffer a bit from reliable sources to draw cards
When you copy a spell on the stack, the rules say the copy copies the characteristics of the spell or ability and all decisions made for it, including additional or alternative costs. That means that, if you cast Mulldrifter for its evoke cost and then you copy the spell with Double Major, the copy is treated as if its evoke cost was paid as well
Rayquaza VMAX (Alternate Art Secret) - SWSH07: Evolving Skies (SWSH07)
The downside is that it can be really hard to gain any advantage if you cant get damage onto their characters without making bad trades (losing a powerful character of yours for a cheap one of theirs) or without completely banishing them
Itll require work to reach its full potential, but that should be worth going for
Most likely, due to the results of Khans of Tarkir 's Delve cards, future expansion sets are unlikely to make Delve a major mechanic, though it may still appear here and there