Great initiative David
I'm going to disagree with Justin (sorry!)
If this mod is based on a trimmed down Forum then it's easier to maintain, because it will be based on very stable and well-tested code, that other Moodle developers will be familiar with.
If this plugin is going to be useful for many Moodle sites (and I believe it would be) then basing it on an existing module makes a lot of sense. A 'typical' admin in a School for example would just want a plugin, that can be easily updated, without having to think about other apps, servers, etc.
Of course, if your aim is to build the mother of all Newsletter plugins for Moodle for yourself (and if it's useful for others so be it) then you might start from scratch. But if you are aiming at wide adoption/use, I'd start with the Forum.
Look forward to hearing other perspective, and also happy to be a tester for this