Regarding the keeping nSequence for future expansion I believe this has
been covered in the specification section of BIP68[1]: For transaction
version >= 2, if the MSB of nSequence is unset, the field is interpreted as
relative locktime, otherwise no special consensus meaning is attached (and
thus free for repurposing in the future). Effectively if the MSB is set,
the remaining 31 bits (out of 32) are free.

Also please note the BIP112 text has been updated with several more
usecases.