A cold ray would make sense if it is 1) AoE, 2) channeling and 3) with very moderate damage output and 4) not freezing.
1: Think of it as a sort of flamethrower, with not necessarily a wide width, but a decent range, so it can affect enough targets on its range to make sense as a directional mass slow attack.
2: The spell has to require a little bit of time to prepare/charge up, then to lock the character where he stands during the whole casting, and then again a little bit of time to unwind/stop the casting. Such specific ability able to be used while running or in hit-and-run attacks would make very little sense.
3-4: at last, in order to make it no direct competitor to the eather/chaos/fire rays, this has to be a low damage output ability, more into mass slowing than killing. And it cannot freeze anything out of the most obvious balancing purposes.
But because of 1), I don’t think that this is technically possible in Grim Dawn. And an Obliteration-like ray turned into cold/slow would make very little sense because of its accuracy and also because of the massive lack of variety and creativity.