Personally I don't think devil force will be a thing. Although even if there is I think there will be two things to consider here:
1.- END is a fire demon. Considering natsu's power it is well likely that END is a dragon slayer as well (hence why acknologia is wary of END). A dragon slayer curse if you will.
2.- END will likely end up being something that separates from the real natsu. So on top of having a fire dragon slaying curse he will have a dragon force too.
It wouldn't be weird that grey with demon slaying magic couldn't deal with END if he is a demon with a dragon slaying curse with access to dragon force.
Also worth noting, even if devil force is a thing it won't necessarily be particularly powerful. Dragon force is the power of dragons. So demon force would mirror that and be the power of demons. However demons haven't yet shown to be stronger than regular humans, at least not by a huge margin. MIra has taken over a number of them and mard geer seems comparable to gildarts at the most (IMO). God force would be more interesting because there is still potential for gods to be potentially as strong as dragons (defeating dimaria took the full potential of shelia, which could easily be ridiculous power in itself, and god slayer magic).
Now, I don't believe any slayer type magic is inherently weaker than the other (it's humans imitation the power of other creatures so it ultimate depends on the power of the human in question) however if "force" is the power of a creature itself then it should only be as strong as that creature.