The first part is a package(s) for handling OpenStreetMap data in convenient form, which will automatically translate between internal and XML forms, communicate with server, and give easy access to all objects. Also, it needs to have some abstraction to give ability to manipulate virtual ways, so-called segments, as suggested by somebody on IRC (#osm-ru@oftc).
The second part would be an OpenStreetMap editor in Tk, utilizing the package(s) from the first part, and the segmented approach.
Another option would be a stand-alone public transport routes editor, as currently editing routes is not an easy task, and it is not visualized enough.
Currently, there are some GIS-related packages for Tcl and Tk, some of them can be used in this project.
AK Some references to related things.