UMLR — Generalization should be allowed to be cyclic and should no be restricted to be owned by the specialized classifier

Currently, UML 2.4.1 requires that:
The graph of classifier generalization relationships must be acyclic
See 7.3.8 Classifier [2]

Generalization hierarchies must be directed and acyclical. A classifier cannot be both a transitively general and
transitively specific classifier of the same classifier.

This constraint probably came from the influence of programming languages on the design of the UML.
This constraint is certainly useful in many domain-specific applications of the UML but it is certainly not useful across all domains.
For example, in ontologies, it is common practice to use circular generalization relationships among classifiers to express their semantic equivalence; see: http://www.w3.org/TR/owl2-syntax/#Equivalent_Classes
The ODM 1.0 includes this approach as an option for using the UML as a notation for OWL1 ontologies — see 14.2.5.11 in http://www.omg.org/spec/ODM/1.0/
A generalization must be owned by the specialized classifier
See 7.3.20:

specific: Classifier [1]
References the specializing classifier in the Generalization relationship. Subsets DirectedRelationship::source and
Element::owner

This ownership constraint prevents using the UML where a generalization between A and B needs to be added without modifying A or B.