The Metric system would be so amazing if we all switched to a base-12 number system. Base 10 is idiotic.
For consistency, we should also switch to base 12 for angles, instead of using 360 degrees, and probably for time as well, having 2-hour long "hours" (12 of them in a day) with 50-second long "minutes" (144 of them in an "hour"). We'd want to call them some microdays or something though, I’m not sure what the natural sounding prefixes would be.
The French concept of making a meter be one 40-thousandth of the earth’s circumference is sorta cute, but clashes obnoxiously with the Babylonian base-360/60 system we use for angle measure. I think the metric system people originally wanted us to switch to a 400-degree circle, but that never stuck (or I might be mis-remembering). If we used base 12 for angle measures, we could make our basic unit about 1.3 centimeters long, and have the earth’s circumference be about 12^6 of them. (I’d be perfectly happy to accept other definitions of a basic length unit though. Being able to measure distances on the globe easily in terms of both angular and distance measures using the same number is nice but not really essential.)
While we’re at it, we should make sure that every digit and the various powers of 12 can be pronounced with one syllable each (needing two syllables for zero, seven, hundred, thousand, etc. is really unfortunate), ideally without any confusingly similar sounds, and abolish special forms for multi-digit numbers. Saying “fifty three” instead of “five ten three” is dumb, and words like “eleven” are terrible. Chinese people (and various others) get a free boost to understanding basic arithmetic simply by having a relatively sane way of naming all their numbers.
Also, whether you write dates as DD/MM/YYYY or MM/DD/YYYY, our calendar is awful either way. The Maya solar calendar was pretty good, but since we’re doing a base 12 system, we should probably have 12 months of 30 days each (six 5-day weeks per month would be my recommendation) instead of 18 months of 20 days, plus of course an extra week of 5–6 holiday days not part of any month.
I think fahrenheit is a bit more convenient than celsius in everyday use, but I don’t especially like either one. But I’m not going to worry about a better system for the moment, because there’s little chance we’ll ever be able to fix the base 10 problem.