Dont know if these have already been suggested but:

[b]Event Hooks[/b]:

on_login, on_newuser, on_logout for user based plugins

on_comment, on_comment_edit, on_comment_delete for comment based plugins.

on_main, on_gallery, on_image for the three most used page types.

on_submit, on_edit, on_upload, on_delete, on_fulldownload for image uploading and editing, would make writing plugins which deal with each process much much easier.

[b]Variables and Globals[/b]:

zpdb to be the main database class or whatever to enable plugins to utilise the database functions that zp uses.

gallery_title, image_title self explanatory

It would be nice to add search, tagging and rss support soon. Also to be able to select your own thumbnail sizes within the admin panel without having to edit files, as well as an on the fly image processing script so that a plugin which requires a different thumb size, etc can call it/the function.

I could go on but I should have been in bed 4 and a half hours ago!

I didn't read this read every post of this thread but I have a suggestion and a feature request:
I'll start with the feature request, I would like that themes use a standard "header" function that can be hooked into.
Next, my suggestion for a hook, if you haven't already guessed, a hook for the header function :D

Thanks,
Codeman

P.S. I can't wait for the plugin hooks! They will make my integration project complete :D

EDIT: Just thought of something else: override-able functions

A plugin architecture is now in the community build. Plugins for Spam Filtering and for Theme Options have been implemented using this architecture.