It would suck if after all this time, the whole series so far, that is, of Natsu yearning to see Igneel again, the dragon would simply get himself killed by challenging Acnologia. Realistically speaking Igneel shouldn't stand a chance, at least alone. If the other dragon slayers (genuine ones, not Laxus) all spit out a dragon, then things certainly could change. Though that still wouldn't guarantee none of them would die.