The docs are a bit confusing.  A few suggested simplifications.

First, introduce type Value interface{} to represent (even if not
compiler-checked) the driver "subset" values (also avoid the
word subset).

// A driver Value is a value that drivers must be able to handle.
// A Value is either nil or an instance of one of these types:
//    int64
//    float64
//    ...
type Value interface{}
and use Value where that is what is meant.

// IsValue reports whether v is a driver Value.
func IsValue(v interface{}) bool
(old IsParameterSubsetType)

// IsScanValue reports whether v is an appropriate driver Value
// for use in Scan.  This is any driver Value except strings.
func IsScanValue(v interface{}) bool
(old IsScanSubsetType)

// Data can control how it is converted to Value by 
// implementing the Value method.
type Valuer interface {
    Value() (Value, error)
}
(old SubsetValuer)

The ValueConverter interface is not getting anything from
its receiver, and it introduces an extra level that is confusing.
Suggest making it a function

// A Converter converts any value into a driver Value.
type Converter func(interface{}) (Value, error)
(old ValueConverter)

Other issues:
"DDL" is mentioned in the docs but not defined (probably remove it, don't
define it).

Address the TODO in driver.Stmt's Close comment one way or another.