Yeah, your open source multimeter must be completed with these feature:
Minimum, since these are already on the market:
Great idea, I don't know how technically difficult it is to make a combined multi-meter and good LCR meter, but if possible it would be nice.
-Dual trigger input like function to function as mini oscilloscope.
Do not want. A strip-chart style display is the most "scope like" feature I need or want from a voltmeter, but I would rather have longer battery life from a non-graphic LCD. Talking is just a silly gimmick 99% of the time. Maybe it can be implemented in a useful way, but adding it as a bullet-point feature is just going to suck.
-Transmit wireless readings such as audio, video, photo, or just plain vanilla reading
-Built in camera, photo and movie viewer (again, skip the video stuff)
-Stability feature to alert if meter is accidentally knock over or moved.
-Timer, UP, Down, Clock, Alarm clock (Talking timer too)
If you can add some of these features without compromising cost, battery life, usability, or form factor maybe, but I wouldn't go overboard on the kitchen sink features. Better to make a top notch meter and add features that support the basic usage. Every feature you add is another position on the range switch, or another entry in a menu, and turning your meter into a full-fledged computer will do a number on battery life. Important features for a meter are things like logging, storage, and USB/wireless links. Maybe some more flexible audible alert: rather than a talking meter, how about one where you can set a threshold voltage (or other value) and it beeps when that level is exceeded?
One thing that might be nice for a rechargeable meter is one of those fancy near-field power links. I don't know how well they work or how much interference they cause, but it would solve the isolation issues, and allow you to use the meter in 'logging' mode simply by resting it in a cradle.
It would be really nice if you could get a useful battery life out of 3 low-discharge AA NiMH batteries. They are cheap, replacements will be widely available for decades, last a reasonable amount of time, and can be substituted with alkaline batteries in a pinch (make sure the charging circuit doesn't try to charge alkaline batteries). Lithium ion batteries are lighter and higher capacity, but hard to find replacements for.