I voted on what I would expect as kind of a minimum, thinking of it more as a free additional gift.
But any of the last 3 options (additional scene, outtakes) sound great and thinking of it, I might change my mind on if I would pay for it, if it contains something like this. I just love Kinsfairlie and am really looking forward to more.
Your idea of merging the worlds is exciting as well. I now am tempted now to read your upcoming xmas book, just to get a glimpse of what has become of Inverfyre.
I have read all your Kinsfairlie series already multiple times and if you are looking for a proof reader on your world guide, I would love to volunteer!