have individual tables, indeces and views that are either relational or document-oriented, or graph- or object-based while we're at it, on a case-by-case basis

have individual tables, indeces and views that are either relational or document-oriented, or graph- or object-based while we're at it, on a case-by-case basis This is already the case. Nowadays, almost all relational databases (except, of course, MySQL) support XML columns. PostgreSQL supports them rudimentary, and DB2 and MSSQL have even special storage strategies and index structures for XML, i.e. for generic tree structures, data-oriented as well as document-oriented ones. Also, abstract data types ("encapsulation", the base of OO) are implemented in these databases, too (except, of course, in MySQL), as well as other OO features such as table inheritation and some kinds of polymorphism.