Well... you could use a combination of existing plugins etc.
But my users is very dumb. So I have to keep it simple.
I my self use cForms and I love it, but for this purpose, it just too advanced.
I've got an "Go ahead", so make this plugin is a certainty.
For now I have a very rough idea of how this is going to work.
I would really love to hear any input, features ideas etc.
1. Keep It Simple Stupid
2. Use of jQuery, to make the build of the form easy and fun.
3. TinyMCE integration, for easy publish of a single registration form.
5. Export of attending people (.csv)
6. E-mail - Telling people what they signed up for and a link for "de-registering"
7. Theme-able + option to use css from WP theme.
8. Gettext localization (.po/.mo)
9. Option for advanced users to insert html, when building the formular.
10. Set "active period", set max people attending.
We are two that will be working on this and the deadline is in 2 weeks.