So I was reading the lost magic catagory on the site and something interesting hit me, "Lost Magic was erased from the history books because of reasons like being to powerful or the side effects were terrible.So I figured the side effects that come with Dragon Slayer Magic must be incredible! So I find it's been so far un-noticed, If you look at the fight between Natsu and Jellal at the toh, Natsu's scales are small and pretty much hard to notice, but in the fight with Zero his scales are all much more pronounced, you can see that they are much larger covering slightly more that then they did before. Another example is Acnologia, the most powerful Dragon Slayer in history, he started as a normal Dragon Slayer, but the more dragons he killed he became stronger eventually becoming a humanoid dragon, having horns wings and a tail, until he was transformed into a dragon by Zeref, this could mean next time Natsu goes Dragon Force there could be a big difference in the apperance compared to the last time he entered Dragon Force. Another side effect is the motion sickness, this could be due to the fact that Dragons have no need for transportation, because they can fly and/or walk there by giving the Transformation from human to a Humanoid Dragon a even bigger possibility to be true, because of the motion sickness.
Please reply I'd love to hear your Feedback and your ideas.