Multi-Keyframe Editing in Dope Sheet/Action Editor

Hello everyone, Now that we're approaching the new animation system, I'd like to propose/request a feature that allows for multiple selected keyframes to be adjusted at the same time.

This would be useful when making looping animations and cycles for example where the last keyframe needs to match the first frame exactly, without the manual labor of having to keep copying and pasting the keyframe every time we make an alteration. Also for animations where the same pose appears many times.

One other variation of this could also allow for difference-based alteration, where I could select two already different poses and make the same alteration to them while preserving the original state of both (Example: Move arm 15 cm to the left, regardless if Keyframe 1 the arm was higher or more to the left than in Keyframe 2, them both move equaly to the left while still mantaining their individual original position as reference). This reminds of some features of Animaide that didn't make it into vanilla blender yet Like AnimOffset where we can adjust the whole animation by altering 1 keyframe, but this time more precise and constrained only to selected keyframes.

Btw I'd like to also remind/ask if possible to add AnimOffset and AnimOffset masks to vanilla blender as well since we have many other Animaide tools implemented already.

Thanks in advance.