As Drupal is written in PHP there is no limit to what you can do with it. You can use a lot of existing modules but you can also create your custom solution to tailor the application to your specific needs. You only need to create a custom module that extends the most popular Drupal hooks: hook_init(), hook_form_alter(), hook_schema(), hook_theme(), hook_install(), hook_permission(), hook_menu()
Integrate Drupal with other systems using the web services API module. They can be used for reading feeds, exchange data between complex systems which generally produce Jason or XML files.
Drupal is in a continue evolution and improvement. Developers contribute daily with new improvements and thoughts. Anyone can propose new ideas, vote existing ideas and also leave comments. Follow the community to see what is going on, see the latest security updates and the new released modules.