I think we should keep it in case we make special anti-crime buildings that are only unlocked with a high safety score. Perhaps some bonus to military XP or special promotions if you get a city with a high enough safety. Or just a bonus to happiness and health. There is lots of potential.
Sounds good. It should also mean that 0 in crime isn't zero in crime but low crime while on or near -200 in crime IS zero.
With the ranges of -200 to 800 it would be something like:
-200 to -101: Protected/No Crime
-100 to 99: Safe/Low Crime
100 to 299: Unsecured/Medium Crime
300 to 499: Dangerous/High Crime
500 to 699: Hazardous/Rampant Crime
700 to 800: Lawlessness
If you want any chance of getting to and staying in the negative then I think 5 crime per pop is way overdoing it even if you are planning to add more buildings as well as the promotions for crime reductive units, and increasing the crime reduction on existing buildings.
Population of 40 (+200 crime) around when Jails come into play right now gives somewhere around 60-100 reduction in crime from buildings. Having or wanting some of the increasing commerce or gold that give crime rate too can probably easily get us up to +150 crime/turn. How much would a Crime Control unit reduce?
In effect probably impossible to get/keep crime below 0 once your capital reaches size 5 (offsetting the -20 crime and adding to +5/turn).
Cheers