Nowadays, there are many frameworks for developing domain-specific tools. However, if we want to create a really sophisticated tool with specific functionality requirements, it is not always an easy task to do. Although tool-building platforms offer some means for extending the tool functionality and accessing it from external applications, it usually requires a deep understanding of various technical implementation details. In this paper we try to go one step closer to a really open graphical tool-building framework that would allow both to change the behavior of the tool and to access the tool from the outside easily. We start by defining a specialization of metamodels which is a great and powerful facility itself. Then we go on and show how this can be applied in the field of graphical domain-specific tool building. The approach is demonstrated on an example of a subset of UML activity diagrams. The benefits of the approach are also clearly indicated. These include a natural and intuitive definition of tools, a strict logic/presentation separation and the openness for extensions as well as for external applications