The problem isn't that he's making a new spin-off, the problem is the writing. We can see that with EZ and 100 Quest, he wastes all the potential of the story, either because he's moving too fast or because he's too lazy to emphasize/work on what's been said.
They set the plot, but after that, we just don't have time. The fights are rushed and rushed, and the plot is quickly over.
Here we see it with the Dragon Gods, who are supposed to be at 100% strength, but they get beaten in a 2-on-2 match, without any real difficulty. If Natsu and co. had gotten stronger, with a PU, training, or something else, it would have gone better, but that's not the case, so there's no real tension.
Now, with the last chapter, Natsu could beat Fire and Flame single-handedly if he were in full serious mode.
So, bringing back characters who would be connected to the Gods, God Slayers, Angels, gods, ANK, and telling us that they are infinitely more powerful than X character, that wouldn't give anything nice to see.
As for ANK, we would need to know if he's a good or bad god. As it stands, he would be a bad god (and damn stupid too). Cursing Zeref because he wanted to resurrect Natsu is okay, but hey, the teacher and everyone in the city who died because of it is a bit of an exaggeration.
Maivis cursed for using LAW, without bad intentions, and to save Yuri, abusing it, and then the same problem of the difference between LAW and Fairy Law or Grimoire Law is probably less powerful, but hey, ANK doesn't intervene there.
And then the same thing, in the end, he's an idiot because he cursed Zeref for wanting to resurrect Natsu, but in the end, that gave Zeref all the time in the world to resurrect Natsu, and once that was done, ANK didn't kill Natsu directly to punish Zeref and made fun of him.
So, as it is, ANK would just be stupid and mean as hell, and on the other hand, Mashima probably hadn't planned anything more for him.