I have a codeigniter script, developed by another developer. It's basically handling the DB connection, then runs the various scripts. From what I can see by reviewing the code, most of what the application does can be done in Joomla without CI.
Once the port is done, I would also need another "library" created. The prior developer basically created the CI interface as a "plugin" system to handle the various types of software it would interact with.
I can explain more to someone who may be interested.