Since default() now has an effect beyond just being a sanitizer, I think it should probably be in ContextHandler, next to optional():

Then, when a context is having field instances selected, the default value is applied:

WDYT, @lpizzinidev?

Hey @lpizzinidev, it just occurred to me that this is a breaking change.

Reason:
default, as a sanitizer, can be placed at any point in the chain, and it only changes the result from that point onwards, which is pretty much the issue with #1057.

// A sanitizer that changes a fruit to `null` if it's value is `apple`
const sanitizer = (fruit) => fruit === 'apple' ? null : fruit;

check('fruit').customSanitizer(sanitizer).default('banana').isLength({ min: 5 })
check('fruit').default('banana').customSanitizer(sanitizer).isLength({ min: 5 })

I still think that your change is a good one, but it will have to be part of v7 (see #990).
There might be more changes needed here though -- but rest assured I can implement them later 😄