The paper "What Goes Around Comes Around" by Michael Stonebraker and Joey Hellerstein surveys about different data models and the associated query languages introduced in the past with a view to prevent repeating the history when inventing new data models.
As mentioned in the paper, there have been 9 major data model proposals:
Hierarchical (IMS): late 1960’s and 1970’s
Directed graph (CODASYL): 1970’s
Relational: 1970’s and early 1980’s
Extended Relational: 1980’s
Semantic: late 1970’s and 1980’s
Object-oriented: late 1980’s and early 1990’s
Object-relational: late 1980’s and early 1990’s
Semi-structured (XML): late 1990’s to the present
It's a good read up to get a grip on how data models evolved and the lessons learned.
Authors predict that XML will become popular as an 'on-the-wire-format' as well as data movement facilitator (e.g. SOAP) due to its ability to get through firewalls. However, they are pretty pessimistic about XML as a data model in DBMS mainly because of its complex query language (XQuery), complex XMLSchema and its having only a limited real applications (schema later approach for semi-structured data) which cannot be done using OR DBMS's. It seems if you don't KISS ;-) (Keep it Simple and Stupid), you are going to loose.