We note that a slow-moving particle will have total energy (sans any potential energy) of
E = mc^2/2 + mc^2/2 + mv^2/2
Let us suppose that all energy of the particle is kinetic or potential energy.
The above equation suggests that the particle is also moving at c along two dimensions unknown to us. There is no known requirement that these dimensions obey special (or general) relativity, or that their fundamental constants are the same as those in our familiar dimensions. The kinetic energy from this motion yields the "rest mass" internal energy of the particle.
Such a situation would mean two things. Mass all by itself is not energy. And there are additional dimensions.
Starting from here, modified as may be necessary, could this be made to work (ie, to be compatible with) our existing physics?
Energy then is always ultimately action or potential for action, not "internal." "New" interactions may exist in the two new dimensions.