The best replacement for at will is right to work.

The best alternatives to at will are rated as following:
★★★★★  right to work
★★★★  just cause
★★★  right to work state
★★★  at-will
★★  right to work states
★★  civil service
★★  for cause
★★  periodic tenancy
★  probationary period
★  due process
★  full broadside
★  licence to occupy
★  beam overload
  contract employment
  just cause employment
  tenant at sufferance
  not at will
  at will
  right work

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Speaking about at will, one probably shouldn't forget about exceptions.

Other search terms related to at will and their ratings are:
★★★★★ exceptions
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at will
right to work
just cause