The best replacement for required is optional.

The best alternatives to required are rated as following:
★★★★★  optional
★★★★  mandatory
★★★  requested
★★★  requisite
★★★  necessary
★★★  addressable hipaa
★★  needed
★★  preferred
★★  require
★★  recommended
★★  credit check
★★  expected rate of return
★  addressable
★  ng-required
★  desired
★  sufficient
  requiresnew
  power available
  deposit
  elective
  npsh available
  fees
  available
  check

Your alternative for required is not listed? Suggest!
Speaking about required, one probably shouldn't forget about freeview aerial.

Other search terms related to required and their ratings are:
★★★★★ freeview aerial
You have in mind a companion for required and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of required in all years. Click on graph for more.
required
optional
mandatory