Pros: same as variation 2 above.
Lose support for required args.
Call()handles non-zero-value defaulting, making explicitly passing zero-values difficult.
Variations 2-3 above have a lot of nice properties.
It goes a little against the principle of "letting things be what they are." If a thing exists solely to be called, shouldn't that thing be a function instead of a struct? At the same time, Go offers better flexibility for defining struct values than it does for argument lists.