Add -AdditionalProperties parameter to Get-ADTreeGroupMember
This would just be to return additional attributes as properties for the returned items
$tree = Get-ADTreeGroupMember -AdditionalProperties displayName
$tree[0].AdditionalProperties['displayName']
Not sure if it would be better to have the provided properties attempt to be retrieved from all object types, or provide a hashtable for each type like -AdditionalProperties @{ user = 'displayName','mail'; group = 'displayName' }