I fancy the idea of gunning Malak with a machine gun myself.

I believe Its safe to assume that Malak will be a Final Boss. Also we can predict that the Trigger Teddies will behave exactly like the Pain Heads. If Glowstick wants to do this collaboration, unless it would not blend well with the game, port the models from Dark Deception to the ILLWILL "DLC"
Something that I do want to emphasize, having a "DLC" will not be a bad idea, it will keep it separate from the main game, giving the impression that ILLWILL does not need to rely on "collabs" back to the DLC, another reason is I would be willing to pay money for it. I got more fun out of this game for $20 that I would out of a $60 AAA with around $100s of "DLC" or buying the second half of the game that that they neglected to finish on time. So I'd bet I'd get more fun out of a collaboration "DLC"