I would really like to see it as well. We recently started taking baby steps into mythic but that means after we finish our heroic clear we are forced to sit 10 people for the rest of the raid and into the next raid window as well. Now I know the content is balanced around 20 man for world first raiding but there should be a point were flex can be enabled after they have had their turn at it.
I am sure fight mechanic balance would cause issues but it would be a great thing in the future to go with the removal of the restriction on cross realm mythic raiding.