I agree RetroMags should catalog pro newsletters, especially considering they're more obscure than the traditional magazine, so archiving them digitally is very important. It's probably a matter of nailing down the publication schedules & histories first though. How many issues, etc. Again, a task made difficult because of their obscurity.
I would like to see fanzine newsletters eventually cataloged as well, but I don't know if the current database convention could be applied - fanzine publication schedules were often very sporadic and numbering/volumization was often inconsistent. Jokingly so, in many cases.
For that matter, I'd like to see general, misc. ephemera cataloged. Pamphlets, catalogs, etc. But I can understand why this would probably be considered scope creep.