Good job Marc, interesting reading!
Yes, shipboard credit and additional savings would be good. I'm in favor of perks that are discreet and not blatant like preferred dining. Why alienate new passengers?
The category upgrades wouldn't work so well for us except on the Mariner (paying for a D and getting a C) as I never have seen any benefit of paying more to have the exact same cabin just to be one or two decks higher.
