Would be great to have the capability of extending Comix using plugins or script (in this case, I vote for python). Plugins/scripts can operate on multiple levels, without bloating the whole software, and this would allow developers to add new features quickly.
Plugins could give developers the ability:
* to customize the interface (add new buttons, menus, side panels for additional infos)
* to customize the picture display (I feel this is important, as it's a missing feature in the majority of the viewers), for example, displaying 1 page, 2 pages, 4 pages, thumbnails in the main view, background color, aligning an image on a certain side, adding texts in the display, etc, could all be done using this interface.
* to customize the user interaction (this goes aside with the previous one), allowing to use the main window to display thing AND interact with them.
* to give additional image processing features, extending the fundamental ones given by you
* to give more control on the images metadata: plugins could add user rating, tags, categories, and also translated captions when reading comics (this can be paired to the custom picture display, allowing to provide a comic with different translations!)
I could go on forever and think about cool features that could be done with plugins :) These are only ideas, in the case you want to provide plugins/script support.
I still didn't look at the code of Comix and I don't know how you developed it, but you can provide, as a start, a basic plugin/script system which hooks the current code calls, giving user the current (GTK?) context to work with. This isn't the best option, but could work as a quick and flexible addition until a better interface is exposed.
Personally, I would go for scripts instead of compiled plugins.
P.S. I just checked out the source and I see it's written with python! Yay! I'll take a look.
Good job anyway, I love this viewer :)