Americans have this stupid idea that if something happens you can blame everyone except idiot who got hurt and his parents that they did not teach him list of "don't do this". Should we care? Hell no!
What can we do to prevent accidents?
If we won't write some warnings then we are to blame (a little). But every mains voltage kit I did had nice paragraph about safety and that is enough for normal people around the world. We are not going forcing anyone to use it.
I think it should be a kit, not a complete product. Then you don't have to bother about some safety issues and certifications. We should not even bother providing enclosures or parts. There are many sources for that. PCBs distributed by some people would be sufficient in my opinion. Good documentation/tutorials plus code is still time consuming. Someone can create kits and distribute them later...
Should we create Arduino shields? NEVER! This form factor begs to be unsafe when it comes to any higher voltages. And I don't really think useful multimeter can be made on (even big) shield.
But... We should use Arduino as a base. As a coding platform and interface to real world it is great. Why? Because you can jump to C or assembly when you need to in any section of your code. Changing default MCU configuration and taking full advantage of its capabilities is easy. If controller would take Arduino code, then we have customization and easy development covered. Arduino would be great connector between some first boards and computer during initial design. We can later jump to ARM or something more powerful, and because they ported Arduino libraries, code can be reused.
There is one more question that needs to be answered. Should we create mains voltage multimeter? Why not make first versions going up to 50V like an oscilloscope? Want to measure something higher? You know what to do
I wrote about analog pins on the board connector, but after some consideration, I think that boards should be as independent as possible with something smart built in. Some digital interfaces only providing ready to read data. Without many connections you can then shield them and isolate cheaply.
For example RS485. You have ability to connect multiple boards and external devices (programmable power supply for example). It is popular industry standard and it is quite fast on short distances and can work kilometer away. Microcontrollers are cheap and every board could have some bits without sacrificing cost that much. A chain would connect to a controller that would only handle data display/logging and user interface to modules after querying them what they are. PC would do the same even through terminal. It should be also easy to provide wireless modules for boards.
Seems that there would be need for some clocking signal from controller to each board so they can be synchronized. My knowledge is rather limited and someone having more experience with more complicated measurement gear should write about how they usually work.