Some wikis (e.g. Inclupedia) may wish to import Wikipedia's logging, page, recentchange, revision, and user data, using the same primary key values for that imported data as Wikipedia, while also having their own data in those fields, starting at, e.g., log_id, page_id, rc_id, rev_id, and user_id 1 quadrillion for future-proofing purposes. This will require a change of these primary keys to bigint unsigned, since int unsigned maxes out at about 2.7 billion.
These keys are going to hit their maxes eventually anyway on enwiki, although it might take a few decades more. Going to bigint unsigned will use up more storage, though.
Another issue with this is extensive core use of intval() on these primary keys. One solution is to create a wfBigintval() global function that removes any decimal as intval does and removes any non-numeric characters. However, then it will return a string rather than an int; hopefully no one does any mathematical operations on these, or it will be necessary to come up with alternative ways of doing those.
I can change old_id to bigint unsigned as well, if you think that would be helpful. Should we just migrate all primary keys to bigint unsigned? See also bug 60962 (T62962), which this will at least partly fix. See also [[mw:Manual:Primary key storage in other fields]].