Allow contrib to add additional stuff to router items.

In some cases it would be nice if contrib modules could extend router items with additional information.

Some of this already works:
- You can use hook_menu() and hook_menu_alter(), to put arbitrary stuff into router items.
- You can use hook_schema_alter(), to add additional columns to the menu_router table.

Problem:
The columns are hardcoded in _menu_router_build() (D6) and _menu_router_save() (D7), so none of the stuff added via hook_menu() or hook_menu_alter() will ever get into the database.

Possible solutions:
a) Allow to add new columns via contrib, and make sure that data added via hook_menu for these additional columns is not discarded.
b) Add a general-purpose "route_options" column, storing a serialized array.

Use cases I can think of atm:
1) D7 has two additional columns in the menu_router table, named 'theme_callback' and 'theme_arguments'. If this functionality was non-existant, there would be no chance for contrib to add it.
2) A new 'menu_parent' or 'menu_parent_callback' could be used to give a hint about a router item's position in a menu tree, to override the automatic behavior of just chopping off the last path fragment.
3) For the Crumbs module I had the idea to allow a 'crumbs_parent' and 'crumbs_parent_callback' callback in the router item, that would be used to determine the parent breadcrumb item.
4) For the Crumbdown module, or other modules with a similar purpose, there could be a callback in the router item to determine possible values for the wildcard arguments. These would be used to build link items to be displayed in a dropdown menu.
5) A similar technique could be used for admin_menu, to automatically generate submenu items for paths with wildcards. For instance, 'admin/content/taxonomy/%taxonomy_vocabulary' could automatically get a submenu with one item for each vocabulary.

It can be discussed whether it is a good idea to put all of the above into menu_router. Maybe it is not.
Still, worth to think about it. And maybe there are other use cases I have not thought about yet.