I am writing a specification for updating pt-online-schema-change. The outline of the changes I want to make is here: https://www.percona.com/docs/wiki/ptdev:blueprints:redesign-pt-online-schema-change
The idea is to make the tool Do The Right Thing, with features such as automatically throttling its operation to avoid causing replicas to lag. Many of the features and improvements are similar to those added to the newly rewritten pt-table-checksum.
If you have any comments on it, please write to the mailing list with your suggestions. Thanks!