It should be possible to specify optional arguments

It is not possible to declare an optional argument: it is possible to define a default value for a nullable argument, but it is not possible to declare it as optional.

This would be very useful in cases where you might want to add a new parameter, but without imposing a change in (stupid) clients that do not automatically add missing parameters as empty tags.

Here is an example (partial) WSDL with optional arguments:

<xsd:element name="message">
    <xsd:complexType>
        <xsd:sequence>
            <xsd:element name="to" type="xsd:string" nillable="true"/>
            <xsd:element name="subject" type="xsd:string" nillable="true"/>
            <xsd:element name="text" type="xsd:string" minOccurs="0" nillable="true"/>
        </xsd:sequence>
    </xsd:complexType>
</xsd:element>