Well wouldn't the proper answer to this is to use what is already in place as a guideline for what comes in the future?
If you're talking about the Spirit of Classic, then it's the contents and mechanics of 1.12. Not 1.07, not 1.13, but 1.12.
Some people are talking about new abilities or balance tweaks for this and that. That could definitely be in a conversation, but one that Blizzard should be considering internally before presenting to us. The same kind of testing they would have done to reach the conclusion that 1.12 is the most stable and agreeable patch to play and start on, despite Vanilla WoW having none of that same experience when it was first released.
If we're going to go down the rabbit hole of flight or no flight; using 1.12 as a guideline easily answers those bigger questions. When you get into details of whether X class should have new itemization to allow them to tank or heal or DPS more efficiently; that will have to be decided upon depending on what they evaluate as 'within the Classic experience'. And that's really all there is to it. It's gonna be complicated, it's gonna have answers that people will hate, but that's the nature of Classic in general. Not everyone loves Classic as it is right now, and that's not a bad thing.
We don't have to go down a slippery slope just to have new content. Blizzard is the arbitrator, not us.
And TBH, I see no harm in people talking about what they want (or don't want) to see. Even if it's not ever going to come to fruition, it's nice to see that some people have an opinion about it rather than everyone just assuming that this community is unanimous about #nochanges or #nonewcontent.