I'll second support for the VIPER System 1 keyboard. It was actually built directly into the RAM expansion, and should work on VIPERSoft BASIC and I believe even the regular BASIC versions without needing a driver or anything.
The other thing that comes to mind is cartridge hot swapping. You should be able to remove one cart and insert another without resetting the system. The software list includes a bunch of BASICarts, which were BASIC programs converted into cartridge format. You need to swap the BASICart and the BASIC cartridge to run them.
And with both of those feature implemented, you could actually load 2000 baud programs into VIPERSoft BASIC. Unlike Blue RAM BASIC, it doesn't have a built-in audio interface, so you need to swap between the VIPERSoft BASIC cart and the regular BASIC cart with the interface. There's not a whole lot of software for it, but Dragon's Castle is pretty cool.
http://www.ballyalley.com/program_downloads/ram_expansion_required/programs_a-h/Dragon's%20Castle%20(198x)(unknown)(2000%20baud)(VIPERsoft%20BASIC).zip
The Music Maker cart includes a tape interface. As far as I know, it's identical to the 2000 baud interface in the BASIC cart.
This is really reminding me of all the surprising things people managed to make this system do--often just one or a few people selling software and hardware through newsletters. I don't even remember how all of it works, but we'll try to figure it out if you want more details. We'll have to think about what would even make sense to emulate. Like, I archived a driver for the Blue RAM Modem, but I have no idea what you could do with it if you emulated one. There is a little bit of light pen software. I also think it would kind of hilarious to emulate the Computer Ear.
http://www.ballyalley.com/pics/hardware_pics/Computer_Ear/computer_ear.htmlYes, someone actually made a speech recognition device! Adam has one, along with some program printouts. (That would be a good video for your Youtube channel, Adam!)