The best replacement for couchdb is mongodb.

The best alternatives to couchdb are rated as following:
★★★★★  mongodb
★★★★  cassandra
★★  couchbase
★★  redis
★★  hadoop
★★  riak
★  mongo
★  dynamodb
★  firebase
★  ravendb
★  cloudant
★  hbase
★  pouchdb
  rethinkdb
  simpledb

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

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



Google Trends

Scale is based on the average worldwide traffic of couchdb in all years. Click on graph for more.
couchdb
mongodb
cassandra