At the moment we can't copy and past berween frames (not entire frame but parts of it), it creates new Layer as usual in 'paint mode' .
We can have another shortcut for this operation (ex. Ctrl+Shift+V)

Probably, the paste should also save the frame value inside, and in case the destination frame differs, ask the user what to do. There are several options what to do:

1. Paste the selection into a new non-animated layer (what we do now). It is not logical at all.
2. Paste the selection into a new frame and merge it(!). The problem is that you cannot move it after the paste without destroying the background under the paste. Gimp solves it with a special type of "Floating Paste", which is extremely weird from the UIX point of view.
3. The third option would probably be the most useful for painters.
   1. When Ctrl+V is pressed a new animated layers is created and the frame is pasted into the corresponding frame.
   2. Then the user adjusts the pasted object as needed
   3. Then just presses Ctrl+E to merge this layer (with only one frame) to the layer below. Basically, finishing the paste operation.

What do you think about it? I guess the most useful options are 2 and 3. And 3 is the most versatile. The main question, should we ask the user about the choice between 2 and 3, or we should just use 3 as the default?

"Paste the selection into a new non-animated layer (what we do now). It is not logical at all."
I wouldn't say it is not logical at all, since it follow the base paste logic in krita. Plus, it is actually useful as it allows to create easily temporary "off-peg" references (which is when you move a drawing temporarily to can use it as reference to draw another frame.. TVPaint has a special feature for this, but I found it easier to do in krita with the current select/copy/paste/transform combo...)

I guess your proposal 3 would be best as it still would work good for off-peg quick paste (might even be better for it :) )

I use option 1 while i paint, that's why i prefer to add another shortcut for this operation.
I like the 3rd option, but i think all the process (A, B,C ) could be in background mode to avoid confusion. Maybe when you paste something with this option, it is already in transformation mode, and when you press Enter it automatically merges with layer below, and user can't even see new animated layer (sounds like a Macros). OR you can choose this in Preferences - to see and control all the process (A, B, C) or not.

I was discussing this issue with @scottpetrovic, @jounip and @Bollebib at the sprint, and I had the following idea:

When you have a selection, you implicitely wish to copy that selection. So while regular ctrl+drag will copy a whole frame, when there's a selection, that would become a copy+paste action, moving only the selected bits to the dropped frame. It seemed to make sense at the sprint, no idea how everyone feels about this now.

Otherwise, the issue with #3 is that if there's no marker frames around it, ctrl+e will become messy.

i second what woltherav said.

I had totally forgotten we already discussed that

A selection copy/drag or move/drag would be great workflow enhancment

If there is an existing frame maybe you should get a warning and option to
1)merge
2)replace
3)cancel