The best replacement for di2 is ui2.

The best alternatives to di2 are rated as following:
★★★★★  ui2
★★★★★  eps
★★★★★  sram red
★★★★★  dura ace
★★★  mechanical ultegra
★★★  super record
★★★  red 22
★★  mechanical dura ace
★★  7900
★★  ultegra di2
★  ultegra
★  dura ace di2
★  sram red 2012
★  campagnolo eps
  super record eps
  sram force

Your alternative for di2 is not listed? Suggest!
Speaking about di2, one probably shouldn't forget about sram cranks.

Other search terms related to di2 and their ratings are:
★★★★★ sram cranks
★★★★ sram cassette
★★★★ rotor rings
You have in mind a companion for di2 and it is not yet here? Propose!



Google Trends

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di2
ui2
eps