"Is there something specific you had in mind?" My understanding is that it can send but can't receive email. Is that true? (Maybe I read it wrong.) If so, then I have in mind: receiving email.
"The second issue of course is what audience is out there to develop software for and what's the motivation of the developer." You're right in that there is absolutely no market out there whatsoever for such an application. But there are plenty of hacks out there who do these things for the pure joy of it, and so I am surprised it hasn't been done already. And since the Apple IIGS was supposed to be all about graphics, I'm even more surprised. Then again, what is currently available was probably written for the lowest common denominator, which makes sense.
"Push comes to shove, find a 6502 assembly manual and make some free reading time on the weekends and see what you can come up with too." I wouldn't go holding my breath waiting for me to do it! (I wouldn't program in assembly 30 years ago, and I sure as all hell won't be doing it now.)
Personally, I'm happy with doing all my internet work on my PC, even including downloading programs for my Apple IIGS systems. I have a CFFA drive in one of the IIGS systems, so its easy enough to do. Actually, I am most surprised to find CFFA drives for the Apple II - my guess would have been that there would be NO hardware development for the system... and I would obviously been wrong again!