I'd add a "method", which defines what delivery procedure to use (this way enabling IM, SMS or RSS in addition to e-mail).
I'd handle reccuring reminders as well internally with the option to nag the reciever til he confirms (so the system automatically sends out the reminder but only informs the caller once the confirmation is recieved).
Don't bother yourself with the message template as all the packages that are going to use the reminder system, will have to decide how to create the reminder internally anyway.
You are missing the differentiation between HTML and plain/text. Maybe there are even more types of content when sending a reminder, but at least these two have to be present so you can send out alternative e-mails. If a package only provides your e.g. with html content, start a converter to store the text content as well.
While we are at it, create a user preference that will allow the user to decide if he wants to recieve reminders in e-mail or text format. Once this works, it should be extended to let users decide, which reminder(types) he wants to recieve via which method in what format.
Last but not least, a small description on how to move from notifications to reminders would be nice as well (unless you see these two as totally seperate pieces of software).
