reference image/video option for subject and background for image to video

When using an image or first frame/last frame image reference for image to video, it would be very useful to add an option to upload a subject reference.

So in practice I have an environment image reference, and I want to add a specific effect to the space, or a specific character. or even a consistent effect/character for use in other generations. Currently its trial and error each time to try and find the correct prompt. Would additional references be possible? For instance a picture of the kind of character I want, or a video of the kind of effect?

Specific example: I rendered a 3d start and end frame image of a room interior. I wanted to animate the trees outside the window blowing in the breeze, or the sunlight casting shadows on the floor, or a person walking through the room.

It took a lot of trial and error to get the trees looking correct, but eventually managed it. I gave up on the sunlight effect since I was burning through credits, and I managed to get the character walking through the room, but on the next generation it was a different character even with the same prompt. I think video references for the effect might have helped, and image references for the character might give some consistency.

Additionally, I wonder if theres a way to specify which area of the image to edit in the video, by a grid ref over the image for example. Whilst attempting to add the shadows and moving trees I lost count of the amount of times I had tree branches and leaves flying through the room nowhere near the windows.