I've been wanting this in the game for a while now, and I think it would be a great addition! Obviously, you can have relative positions when it comes to coordinates for entities, including armor stands, but I think that you should be able to add a relative position when positioning an armor stand with a command, so that you can change an already existent pose, instead of having to make a completely new command to change it. The ~ parameter would still work in this scenario, so it would be easy to use because that's what the symbol for relative coordinates already is.
What I mean is this:
So instead of setting the left arm to be at 100 out of 360, it would add 100 to the position the armor stand is already in. This would allow you to make animated armor stands very easily by having a command block that repeats for a bit to change the armor stands position to something and having it do it slowly, with a factor of like ~2f so that it only moves by 2 every tick, but if you have the command running for a couple seconds, the arm will have moved gradually from it's starting position to the next position in a very smooth fashion. I think this would be a great addition and would allow for a lot more creative freedom in adventure maps or just having fun messing around!
If there is already a way to do something like this, I could not find it anywhere on the internet and have been searching for years and there has never been a simple vanilla solution I could find. I know it seems like a minor change, but I think it would improve a lot of things and map builders would be able to make more engaging and realistic experiences for their players!