There are no immediate plans for MongoDB specifically, but in 2.1 we are introducing Elastic Search to replace SOLR, with likely follow-on investigations into use that for "indexes" or similar. Still early stages, but I could imagine approach is to keep master information still in MySQL, but put index in Elastic Search. Browsing etc hits the index, so those operations would be faster. Master data is still protected by a solid transaction safe engine. MongoDB is tranditionally known as good for reads, but not good for writes. Using it for indexes might also be a good fit then as well.
We are also doing work around the database APIs as a part of Staging work to improve abstraction layers internally. That may open up more possibilities to replace the database engine in the future as well. So we are edging in the right direction, but have to deal with issues like join queries across tables (more problematic if one table is in MongoDB and another is not). So a bit more thinking to be done there.
