Line item Entity

My initial development of a line item type

There are three issues that is see and are all related to Drupal core.

1.  Entity reference configuration requires the selection of the target entity and bundles, there is no option for all bundles. The line items field on orders needs to be able to reference all line item types; products, shipping, etc.
2.  The same applies to the the product line item type which needs to be able to refference all product types.
3.  The config is only able to create the line items field on the order bundle, if a user creates a new type of order type from the UI it will not have the line items field on it which will be required. (This also applies to the price field on products)