When you create a new document set, you are presented with a modal dialog such as the following:
I want to have this kind of "New Form" (modal) for my custom content type (with custom form).
NewFormUrl property for a content type, where I can redirect the user upon creating a new item from my content type ( http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.spcontenttype.newformurl.aspx) - this is not the same, because I think it is nicer to stay on the same page.
New > MyContentType > User is presented with a form as in the Screenshot above > User enters some metadata > Item based on MyContentType is created with metadata entered by user.
Also when creating my own content type new form, would I need to recreate everything Microsoft has done so nicely for the Document Set (i.e. Save Button, Check-In, Copy/Paste)?
EDIT: And then there is that: How to Open a List Form in a Modal Dialog Box