It should be possible to add icons to ComboBox controls and the item delegates within, just like can be done with Qt Widgets QComboBox, without having to adjust the contentItem or itemDelegate.

API-wise I suggest something similar to how currentText and textRole behaves. textRole should perhaps default to DecorationRole. A displayIcon property would default to whatever icon is assigned to the current item but can also be overridden to show a custom icon or show a static icon independent from the current item.

The difficulty lies in how to interface the QIcon-based DecorationRole of a model with QtQuick / QtQuick Controls Icon, and also how to make icons possible in models in a purely declarative way (both as JS Array and ListModel).

API examples

A ComboBox which contains entries with icons and takes whatever icon the currently selected item has (icon API also purely fictional)

QQC2.ComboBox {
    textRole: "label"
    iconRole: "icon"
    model: [
        {label: "None", icon: "dialog-cancel"},
        {label: "All", icon: Qt.icon("qrc:/icons/all.png"}
    ]
}

A ComboBox which shows only a static icon independent of the current item

QQC2.ComboBox {
    displayIcon.name: "view-filter"
    model: ["All Schemes", "Light Schemes", "Dark Schemes"]
}

A ComboBox which has a custom dynamic icon based on the current item

QQC2.ComboBox {
    textRole: "label"
    iconRole: "icon"
    displayIcon.name: currentIcon.name + "-symbolic"
    model: [
        {label: qsTr("Red"), color: "red", icon: "note-red"},
        {label: qsTr("Green"), color: "green", icon: "note-green"},
        ...
    ]
}

Usecases for icons on ComboBoxes include (but are not limited to): indicating "orientation", application icons (e.g. chosing a default application or handler), country flags in locale settings.