On Mon, Feb 1, 2016 at 4:55 PM, Pieter Wuille via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:

* The coinbase scriptSig gets a second number push (similar to the
current BIP34 height push), which pushes a number O. O is a byte
offset inside the coinbase transaction (excluding its witness data)
that points to a 32-byte hash H. This is more flexible and more
compact than what we have now (a suggestion by jl2012).

So, the script sig is "<height> <offset> ..... <H>"?

Why is this just not the offset in the extra nonce?

> A significant design consideration is that if arbitrary data can be
> added, it is very likely that miners will make use of that ability for
> non-Bitcoin purposes;
I agree with the concern, but I don't really understand how this idea solves it.

It could be enforced that the data in the coinbase witness stack has a fixed number of entries, which depends on the block version number. Version 5 blocks would only have 1 entry.

This would mean a soft-fork could be used to add new entries in the stack.