We have been working on a general-purpose resharding for over 3 years, but have yet to release it to the open source community: it's very hard to do it well.
But our customers get a sharding scheme that best suits their business needs, including fully automatic shard management and data re-balancing. I submitted a talk about the technology and know-how behind this to Percona Live 2017: https://www.percona.com/live/17/sessions/best-practices-appl...