The best replacement for capers is green peppercorns.

The best alternatives to capers are rated as following:
★★★★★  green peppercorns
★★★★  caper berries
★★★  caperberries
★★  non pareil capers
★★  olives
★  lebeau
★  peppercorns
★  juniper berries
  capers in brine

Your alternative for capers is not listed? Suggest!
Speaking about capers, one probably shouldn't forget about salmon.

Other search terms related to capers and their ratings are:

★★★★★ salmon
★★★★ chicken
★★★ tilapia
★★ smoked salmon
salmon recipe
chicken piccata
fish recipe
lemon sauce
lemon butter sauce
red snapper
salmon pasta
You have in mind a companion for capers and it is not yet here? Propose!

Google Trends

Scale is based on the average worldwide traffic of capers in all years. Click on graph for more.
green peppercorns
caper berries