There is currently no way to add a field type that uses bigInt, although bigInt is used in a couple of places within the CRM Framework for core processes. Please expand its use to custom attributes as well. From an integration standpoint, many systems use numbers as keys as well as the ability to do large numbers within the same Framework. From a database perspective, indexes are faster when it comes to numbers than text fields, including overall performance. Current suggestions of just using a Text field are not adequate from an indexing perspective as well as a sorting perspective.