The best replacement for imp is succubus.

The best alternatives to imp are rated as following:
★★★★★  succubus
★★★★  voidwalker
★★★  impdp
★★★  observer
★★  piglet
★★  felguard
★  nimp
★  doublelift
★  incinerate
★  pret
★  felhunter
  angel doodle god
  declassified
  squirrelmail
  isc
  fel imp
  wrathguard

Your alternative for imp is not listed? Suggest!
Speaking about imp, one probably shouldn't forget about glyph of demon training.

Other search terms related to imp and their ratings are:
★★★★★ glyph of demon training
★★ imp
You have in mind a companion for imp and it is not yet here? Propose!



Google Trends

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imp
succubus
voidwalker