"Link to Wiki Page" or "Link to Wiki Attachment" are very helpful. But its only available to Pages / Attachments of the same Wiki / same Community. From my point of view it would be helpful to enable this function to select Wiki Pages / Attachments in other Communities.
Such links should have a special design element that shows that it is possible that the user have no rights to see this (e.g. a sign with a exclamation point)