I've had this theory I've been pondering for some time now. I've heard only Dragon-trained Dragon Slayers are at risk of Dragonification. Prior, I thought Laxus and Erik were at risk of becoming Dragons, but because Dragons didn’t train them, Wiki users dismissed this.
Dragon Force may be involved in the Dragonification process. Let's brake down the different Dragon Forces.
- First Generation: those who have been taught their Magic by a real Dragon, certain conditions must be met in order to enter Dragon Force. Unless they reach a certain level, the only one known so far would be to eat substances with powerful magical properties, preferably those somehow related to the element they can control and consume, like when Natsu consumed Etherion and Jellal's Flames of Rebuke or when Wendy consumed Ethernano-contaminaited air; while other, unrelated substances, like anti-Ethernano (Magical Barrier Particles), can be eaten as well, which have been described as being similar to "poison" for the user, causing heavy after effects, such as pain on activation and extreme exhaustion after use.
- Third Generation: those who have not only been taught by a Dragon but also have Dragon Lacrima implanted into their body, are capable of freely entering Dragon Force. However, the appearance of their Dragon Force is slightly different from the previous generations, for instead of actual scales, a scale-like design appears on their face and body.
- Second Generation: those who are like Third Generation, except they weren't trained by Dragons. In the images shown, most Second Generation Dragon Slayers enter these transformations to use their Dragon Slayer Magic. Otherwise, they'll resort to their backups. For Laxus, it's Lightning Magic. For Cobra/Erik, it's Sound Magic. For God Serena, it's whatever he used to make short, easy work of the four Wizard Saints before he used activated his Dragon Slayer Magic. Laxus howewer later taught himself to use it without transformation. If you need proof, everyone was surprised by seeing Sting and Rouge use Dragon Force at will, though 3rd Gen is supposed to have the powers of both 1st and 2nd gens. While Dragon Force is the ultimate stage of the Dragon Slayer, Laxus and Cobra's transformations aren't really different from non-Dragon Force 1st Gen (Laxus is simpy more powerful than Natsu and Gajeel, and Cobra uses his hearing to his advantage), as it doesn't enhance their strength and speed. It's more like a mutation then a power-up. So 2d Gen's transformation is likely some side effect of their Dragon Lacrimas, which disappears after a certain level is reached, but not Dragon Force.
3rd Gen can enter Dragon Force because they apply the power from their Lacrimas to the power they already had.
That said, I'll get right to the point. This theory is what I think would happen if a 2nd Gen, or even a 3rd Gen, ate substances with powerful magical properties such as Ethernano or even God Slayer Magic of their element. Granted, Natsu didn't enter Dragon Force when he ate Zancrow's Divine Flames. He did, however, deliver a power spell, the epic Dragon God's Brilliant Flame, that one shot Zancrow and left Natsu exhausted. My theory, considering how much Magic power they can contain and how natural their transformations are, is if said 2nd Gen were to eat Magic beyond his/her measure, deliver a powerful attack (like Dragon God's Brilliant Flame or when Natsu ate Etherion and before he started choking, he roared and surged with power, slamming the floor), then his/her Dragon Lacrima would then crack and he/she would suffer a nose bleed and seizure before going unconscious. As the 2nd Gen sleep, the Dragonification begins. I pictured it to be some kinda more graphic then how Zirconis described Acnologia's Dragonification. Like a David Cronenberg body-horror type Dragonification.
It'd start from where the Lacrima would be located. The Magic from the cracked Lacrima would "ooze" and spread throughout the body, mutating the host's DNA. The outer skin would peel off with Dragon scales growing in its place. When it reaches the head, the hair falls out, the human ears and nose fall off, and the teeth fall out with fangs growing in their place. When it reaches the hands and feet, the nails fall out with claws growing in their place. Soon Dragon features will grow out of within until the 2nd Gen stops being human all together. I doubt he/she will grow into a full size Dragon but maybe the size of Cubellios when she unleashed her wings, or a Wyvern, or at least half the size of Igneel and Acnologia.
While a 1st Gen Iron Dragon Slayer like Gajeel and maybe even a 1st Gen Poison Dragon Slayer, who can eat poison, could enter Dragon Force by eating Magical Barrier Particles, a 2nd Gen Poison Dragon Slayer like Cobra or a 2nd Gen Iron Dragon Slayer would suffer the 2nd Gen Dragonification. If other 1st Gen could enter Dragon Force buy eating the anti-Ethernano, than the 2nd of that element would suffer from 2nd Gen Dragonification. God Serena ate Ethernano or Etherion, because he has 8 Lacrimas, his seizure would amplify by 8 before slipping into a coma and dying. However, if he ate Divine Flames, only his Purgatory Dragon Lacrima would break and he'd become a Purgatory Dragon. If he ate Sky God Slayer Magic, only his Gale Dragon Lacrima would break and he'd become a Gale Dragon. Sting and Rogue probably wouldn't get this because of their antibodies.
My thoughts anyway.