The best replacement for thresh is leona.

The best alternatives to thresh are rated as following:
★★★★★  leona
★★★★  blitz
★★★★  lulu
★★★  nami
★★★  sona
★★★  blitzcrank
★★  annie
★★  alistar
★★  lucian
★  taric
★  nautilus
★  janna
★  soraka
★  zyra
★  cato
★  zac
★  jinx
★  karma
  thresh
  ap thresh
  fatal1ty
  zilean
  hecarim

Your alternative for thresh is not listed? Suggest!
Speaking about thresh, one probably shouldn't forget about relic shield.

Other search terms related to thresh and their ratings are:
★★★★★ relic shield
★ adc
You have in mind a companion for thresh and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of thresh in all years. Click on graph for more.
thresh
leona
blitz