Also, while some people might not consider my comment above about Blizzard updating the API as useful, consider merely three examples of changes made during the original game:
- The addon Decursive was forced into a significant rewrite. Originally healers could bind one key, then spam it to remove whatever their class could cure. Healers did not need to know whom they were cleansing, nor did they need to know which debuff. It made at least one boss mechanic in Molten Core trivial. It removed most of the decision making process, and so Blizzard forced an alteration to what we see as Decursive today.
- The chat channel known as RAID_WARNING did not exist in classic or vanilla WoW. It was solely the creation of an addon called CTRA (CT RAid Assist) and it required users who wanted to either sent to the channel, or read the messages sent, to have CTRA installed.
- Speaking of CTRA, inter-addon messages were added as part of its functionality. The hidden addon-to-addon chat channel we know today was added by Blizzard because it was such a good idea. In the original form, CTRA simply created a chat channel, set the join privileges to anyone with CTRA, and voila. It was a normal chat channel, and all you had to do to see it was type /join CTRA and enter the correct password.
I am certain there are multiple examples of addon API that Blizzard could adapt from the modern version into Classic, without breaking the look or feel of the gameplay. There are examples of addon creation other than Decursive that Blizzard never intended as possible. These are the non-breaking changes I'd fully, 100% expect in Classic.
There are other API commands that might be brought over as a package dump, yet remain inert because other functionality won't exist, such as the collections API (pets, mounts, heirlooms, toys), but I am less confident in that assertion.
And what about the combat log API, which was totally rewritten midway through TBC, in content patch 2.4? Blizzard could make a really good case to use that version over the prior version. Having access to threat APIs among other things would certainly be useful rather than the pure guesswork authors used at first. Yet again, if Blizzard did use the newer combat log, it won't break anything regarding the look and feel of Classic.
Last but not least, people are losing sight of one core idea regarding Classic (or any version of WoW). While Blizzard allows addons to exist, they are not a requirement. I totally believe that Blizzard will allow addons on Classic, but just like all versions of the game, the look, feel, and basic functionality of how we use the user interface will be entirely up to Blizzard. The notion of Blizzard using the old, broken, incomplete API of vanilla in Classic just because it is Classic is ridiculous.
No, Blizzard will use an API for Classic that retains the experience, yet follows what they deem acceptable functionality. Do not fool yourself otherwise, which is why I am not jumping onto a private server to write code. What would be the point if all the work I do gets undone the instant the real API is revealed?