There are several major schema changes in the planning and early development stages:

• T166732 will replace columns such as rev_comment, ibp_reason, img_description, and so on with id fields referencing a separate comment table.
• A similar change will replace columns such as rev_user/rev_user_text, ipb_by/ipb_by_text, and so on with id fields referencing a new 'actor' table.
• Content model and format fields will be changing to ids too.
• And, of course, MCR will drop a number of revision fields (rev_text_id, rev_len, rev_sha1, rev_content_model, rev_content_format), moving them to a 'content' table with multiple content rows corresponding to one revision.
• There's also a proposal, currently unresourced, to change various namespace+title fields (e.g. pl_namespace and pl_title) into id fields referencing a central namespace+title table.

I don't know if there are any statistics as to how many tools would be broken by these changes, or if there's even any way to really determine that.

It has been suggested that these changes might be eased in Labs by providing "old" versions of the views for the affected tables. For example, while the revision view would presumably change to match the schema changes, an old_revision view that pulls in the revision, user, and main slot data in a manner similar to the current schema might be provided (temporarily). This would allow existing tools to simply switch table names to keep working while their code is being updated to the new schemas.