I think that a plugin system is interesting. I'd also be interested in
seeing a refactorization of the code, especially with respect to
implementing a templating system. Mantis is a bit crufty.
As for plugins, for maximum flexibility a plugin could simply be a
wrapper for a function call that was called through hooks defined in
functions. By registering for a function (say authorization) you could
pick up and handle all authorization or modify an existing authorization
action by changing incoming or outgoing parameters.
Happen to think the plugin system should come after a real code cleanup
but that is of course just a personal opinion.