The problem with Mutate is that "attach to non-human" is baked into the rulings for the keyword. Most of Venoms famous nonsense involves taking over humans so there's a clash there.
Some X-Men character having "Mutants in your hand have Mutate, where mutate is its Mana Value minus {2}. When this creature mutates, draw a card." Would be sick though.
They could always have a separate Symbiote card as Venom is, himself, a unique character. It might be fun to have an Eddie Brock card that melds with a Venom's Symbiote card to make Venom?
As someone who loves to paint venomized versions of characters I agree that a mechanic like that would had made more sense (but I do love the fact that he's Golgari).
That's good, that's good! I had always imagined the symbiotes would have mutate and something like "Symbiotic — this creature can't attack or block unless it's mutated."
Comments
Otherwise yeah it'd be a slam dunk.
Or something similar enough between Bestow and Mutate.