In order to simplify arithmetic and to only have a small set of numerals...place value is absolutely essential.
We would also need a placeholder of types for nothingness, so a "zero" is needed.
The numerals shouldn't be that difficult to write...or maybe one or two really difficult ones that mean like 1 trillion or something just to screw with people.
But keeping it simple as in lines and curves/loops as in our system is key.
By the way, if we have a base 20 system, wouldn't we have to think of more digits? We'd need a digit from everything from 0-19.
Or do you mean big numbers could be expressed with less digits?
One last thing I could think of would be maybe a simple manner to indicate exponentiation, so you have easy access to factors of the base and could probably reduce notation a bit and thus arithmetic a bit.
Maybe like a dot, or a dash, or some cool thing on top of a number for it's first two base factors.
And now all of this crazy number system talk has made me hungry...so I'm done rambling. But, try to come up with one. I've thought about this many times before but never followed it through.