I feel as if it is going to be blood related.
It seems like all lost magic has consequences if overused. If the Great Tree Arc is overused, the user turns into a tree. If first generation Dragons Slayer magic is overused the user becomes a dragon, or at least they would have if the dragons didn't intervene.
No Dragons were preventing Dragonification in second generation Dragons Slayers, and I doubt second generation Dragon Slayers would ever turn into a dragon if they overused their magic. But, their must be some sort of consequences if they over use their magic. Laxus most definitely pushed the bounds of his magic, and it seems like he is about to pay the price.