The best replacement for sage mode is eight gates.

The best alternatives to sage mode are rated as following:
★★★★★  eight gates
★★★★★  mangekyou sharingan
★★★★  sharingan
★★★  kyuubi mode
★★★  nine tails mode
★★★  rinnegan
★★  curse mark
★  sasuke mangekyou sharingan
★  chakra mode
★  gear second
★  susanoo
★  sasuke taka
  kaioken
  tailed beast mode
  bankai

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Speaking about sage mode, one probably shouldn't forget about kyuubi.

Other search terms related to sage mode and their ratings are:
★★★★★ kyuubi
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sage mode
eight gates
mangekyou sharingan