I'm so glad to hear you're developing a newsletter plugin. Us WordPress bloggers need something like this desperately. As far as features go for your newsletter plugin, some core functions I'd like to see are:
* the ability to place the subscribe form on any page within the blog. A small email field and subscribe button that could be placed on the widget bar might be kind of cool also.
* have the plugin log all the email entries entered and use that list for the newsletter.
* have the ability to easily edit the list if anyone should unsubscribe. Some automation for this would be a bonus. For instance, if someone does indeed unsubscribe, then that email address would automatically be deleted from the email log. But if we have to do that manually, that would be ok also. I understand this would be a first release for the plugin, so I'm not expecting a ton of fancy features. Being able to edit the email log is a must though.
* Optional fields that would let subscribers enter in their name would be cool. That way we could have the option of using their name within the newsletter (i.e. a mail merge type function.)
* Have the option to have the newsletter be in either in text or HTML format.
* Have the ability to insert images, links, formatting (i.e. underline, italics, bold, etc.). Basicly it would be the same sort of options you get when writing a blog post. If you do include the ability to insert images, please also include some control over the image alignment, and the type of text wrapping that could be used. As far as inserting links, please make it possible to have the links show either as full URLs or simple text.
* Have a field where we can easily insert a URL to a logo image for the newsletter. This would automatically place the image at the top of the newsletter in whatever alignment we choose (left, center, or gith alignment). .
* have something that can detect email bounces. I think it's pretty likely that people will type in the wrong email address. Perhaps it would be a good idea to include an auto-responder that would send out a message with the instruction to verify their email address.
* Allow for the ability to save drafts of the newsletter. Something similar to what we have when we write blog posts would be good where we can Save, Save and Continue or Publish.
* A scheduler function to automatically send out a selected newsletter that we've already created.
Those are the ideas that I have. I hope you'll consider at least some of the suggestions. Thanks!