There is no way to change look and feel of the Explorer window which displays current folder in the Outlook. But, Inspector which displays the content of the opened item for some item type can be changed. More accurate, the default one can be prevented to show, and any other Windows form can be displayed instead of.
There is a supported way to make some new form for displaying item or to change existing one. But this can’t be done programmatically. Furthermore, user can always change form again by changing Outlook properties.
You can make brand new Message class which is bounded to Outlook item, but there is no way to introduce brand new Outlook item. It is possible only to add a new one which is inherited from existing one, for example: IPM.Note.MyNote from IPM.Note.
New item can be displayed by a custom form as well.