Hm, I think that’s a good theory, as Igneel is the only one from Natsu’s past that we have heard of. Anyone else who appears to be E.N.D. won’t have had any foreshadowing like Igneel has had. Like you said, Igneel is a fire dragon and is very strong. It would be a difficult task to kill oneself as well, explaining why Igneel couldn’t defeat him. Though, while I think it’s possible that it is Igneel, I’m inclined to think that it might actually be someone else.
Firstly, we recognize Igneel as a dragon and he’s taught Natsu dragonslaying. I think it would be strange for them to be suddenly like, “Nah, he’s actually a demon” and that would mean that Igneel would have been teaching him demon slaying. Secondly, Mashima has done the thing where a person isn’t mentioned until they appear. That happened with Lyon, Lucy’s father, a lot of people from the tower, and with Gray’s father. So, even if there hasn’t been any foreshadowing for any one else, it’s still possible to have someone new from his past appear. After all, people don’t mention a lot of things from their past until the come up and cause problems in real life, too. It’s not that unrealistic.