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I didn't see a location to add suggestions for Spine, so if there is a better place, please let me know where and feel free to move this there. I'll update this as I get to know Spine more.

  1. Graph editor that isn't key to key, but encompasses all the keys over the whole timeline (like Maya or 3dsMax)
  2. If not a full graph editor revamp, maybe make it so when you select a few keys, each curve has a color, maybe a graduated level of grey even. This way it is easier to track which is which, when you have several selected.
  3. Different curves for X/Y for translate, in graph and dopesheet. This is one of my biggest frets
  4. "Flip Bone" to so that tricks like scaling are not needed.
  5. "Save Iteration" Speaks for itself really.
  6. Parental switching! Right now the only way to do this is with transform constraints, which do not allow for multiple switches. Would be nice to be able to switch between world, lock to head/hand/hip/etc.

MORE TO COME!

Thanks for sharing your suggestions! FWIW, you can see what we have planned here:
https://waffle.io/EsotericSoftware/spine

1&2) We are finishing up version 3.7 (audio and other things), then version 3.8 will have a curve editor.
3) Separate keying for x/y is planned. You can subscribe to the issue on GitHub and get an email when there is progress.
4) Scaling to flip isn't really a trick. I'm not sure it would be improved with an explicit flip.
5) Note that Spine saves backups periodically and when you save. It uses a numbering system to organize the backup files, but it sounds like you want to save a copy with a meaningful name. Save As does what you want, I think? Maybe you are asking for Save Copy? That would do the same as Save As, except it wouldn't change to the newly saved project.
6) There are many benefits to knowing the hierarchy. It allows for assumptions/optimizations and some hierarchies are not valid for some features. Allowing the hierarchy to change increases the complexity of a number of things, which is why we chose not to allow it. You can simulate multiple reparenting with multiple transform constraints.