Okay theory time, I think its pretty obvious what the endgame is going to be. Ragna is a sword. He is completely a sword. His way of thinking is that of a sword. He can only swing himself at full power like a sword. The silverware princess, sees a sword when she looks at him. HE CAN TURN HIMSELF INTO A SWORD. Heck the first thing he does is throw himself at Crimson because he doesn't know how to go about hunting the dragons - other then smacking them really hard. I would like to call back to the very first chapter, he only views himself as an extra, Leo is the true prodigy - a master swordsmen in the making. Remember Leo wants to become the strongest, she fights dragons because they are strong, who can follow her mad quest? I think the answer the future Ragna was looking for, is someone that can truly wield him. If they can come to understanding; this time they might be strong enough to defeat the dragon god.
I think the relationship between Leo and Ragna is between sword and swordmaster.
TL;DR: Ragna Crimson is a story - I think - about the making of a legendary sword. Though with that said I still can't place where Crimson fits in that dynamic. The title is "Ragna Crimson" afterall.
Lol, his sword couldn't handle the power being channeled into it. Simple solution, when he's ready he'll become the sword that can channel that power.