@Paul-Nebeling I believe we've already seen the cost. Rehzel was forcibly converted to Extra Mode, and it transformed him into a demonic deity. Beforehand, he was actually willing to pursue reconciliation with the elves, but afterwards he was singularly focused on claiming the world tree. His true goal was really just to have the right to die under its branches, but his transformation shut out his ability to consider any alternate methods of accomplishing that, or any of his other desires. It's like how when Krena was carried away by limit break and was hyperfocused on her goal of fighting a powerful opponent, without being able to think about whether or not she should.
Allen is in hell mode whilst remaining... passably sane, and the demons have talked about stepping through your extra gate, so I think there's a way to safely enter extra mode. The issue is that you can only do it yourself; having it activated for you results in your will being consumed by your own skills, which is why the Gods can't help. In a way, even though it was facilitated by the Gods, Allen entered hell mode under his own will and resolve, which is why he's able to control it. I believe Krena for example could enter Extra mode by learning to view limit break as just a normal part of her, and shattering the restrictions normal mode places on it through sheer force of will. In extra mode, limit break would probably just be a lv 7 skill that can be toggled on or off at will, just like how if Allen was in normal mode, his extra skill would probably be one of Quick Summoning (with limited duration), Deputize (one use per day), or maybe King Me.
One thing I find potentially interesting; the extra skills seem to be randomly chosen from a pool of potential lv 7/8 class skills, as not everyone with the same talent has the same extra skill, but they're always relevant to your class. However, Drops of God is likely a cleric/saint class skill and monk wouldn't get it; if he chose the monk promotion, would have have gotten a different extra skill? Similarly, Sophie's grand spirit summoning is clearly within the "spirit user" class line, and she was not actually originally a spirit user - the direct promotion of spirit wizardess is likely spirit arch-wizardess, but she chose an alternate branch. This would mean grand spirit summoning was a potential extra skill of a class she could potentially promote into, but if she had gone into extra mode as a spirit mage, she never would have gotten it.
