[Imgema]

Any plugins maybe? Or a different video player addon?

I would like to see things like CRT scanlines, blurring/sharpening, composite filters, etc.

[Klojum]

That's what I would consider outside the scope of Kodi. Some things you could do on-the-fly, like gamma, brightness, contrast saturation, sharpening and possibly blurring using ffmpeg, but adding actual effects like CRT scanlines would basically require re-encoding of the video. And Kodi will not be going down the road of Snapchat filters.

[Lunatixz]

A static image or gif as an overlay is possible... and there are a number of Kodi shadertoy demos. It might be possible to create a binary plugin to use a shader like below. However, I'm not sure both shader and player layer can exist together... https://www.shadertoy.com/view/MtlyDX

Technically you could also create a 100% python overlay that creates a dynamic crt scanline effect... It would require a considerable amount of time and knowledge to complete! With the right image assets... Possible

As for your question no compositing, that's not possible.