On 11/16/2015 07:42 PM, Peter Todd via bitcoin-dev wrote:

Sequence is used for opting in as it is the only "free-form" field
available for that purpose. Opt-in per output was proposed as well by
Luke-Jr, however the CTxOut data structure simply doesn't contain any
extra fields to use for that purpose.

What is wrong with using they same scheme as sighash_single?

If input 0 has nSequence < maxint-1 then output 0 is replaceable.

For fee bumps you would just stick the change in position zero and reduce the value.

You get FFS functionality without the hassle of addition other inputs.