Currently, the plugin class loader can't find classes which are in other uploaded plugins. It would make life so much easier if it could. Looking at the code, it seems like the solution would be to have the PluginClassLoader have a reference to the PluginManager and let it check through all the registered plugins if it can't find the class it's looking for. That said, there's currently no guarantee what order plugins are loaded in, so it might be necessary to add a <dependency pluginKey='my.plugin.key'> into the atlassian-plugin.xml descriptor, enabling the PluginManager to delay loading plugins until their dependencies have been loaded. Gets tricky if there are circular dependencies, but that's the developer's fault.

Any thoughts?