Well, this ended up leading in different directions than I expected, so I'll be more specific about my opinions here.
I like the idea of the main (if ...) semanics being just another equality check or dynamic type check: "Is this nil?" If falsiness overlaps with multiple other dynamic types, then we end up having confusing crosshatching where one extension wants to do X with any falsy value and another extension wants to do Y with any list.
Secondarily, I also see some benefit in distinguishing between () and #f, because then it's possible to dispatch on whether something is a list or a boolean. But I'm also happy if we don't have booleans at all, because then "Is this nil?" can just be a special case of "Is this a list?"