This is being asked for consideration for a few reasons:

1. for better or worse, this can be what current combobox patterns are trying to achieve - a input field that allows for someone to submit a value - but also a popup where on activation of a choice something happens on the page / results are automatically updates / an action performs.
2. the line between when something is a menuitemradio/checkbox vs an option/multi-selection option can be rather grey - and arguably for multi-select, menuitemcheckboxes with their clear checked/unchecked state can be more understandable than listboxes > options where screen readers have each made their own decisions on when to announce selected vs unselected - under the initial assumption that selection always follows focus.
3. I could create a combobox that opened a dialog and then that dialog contained a menu within it... there's nothing preventing me from doing that now and it'd be "valid" though a silly workaround to mitigate against the current lack of "allowance" to directly open a menu from a combobox.