Originally Posted by Dagoro
any guesses as to what was it that Makarov showed Gildartz ?
Fairy related, Dragon related, legendary weapon ?
I think it's obvious what he saw.
Sadly, I'm not even joking. I believe that it really is a hidden power source that emits friendship and rainbows, which is naturally buried underground (or part of the ground) and Mavis built the guild over it as a means of protecting/controlling it. I'd also wager that it's somehow connected to "hope and friendship=you can't die" power of the island in some way.
If Hiro really wanted to make it interesting, he could
borrow some plot devices from Naruto and Inuyasha, and have it be the remains of some ancient hero who once fought Zeref and sealed himself and Zeref's Yang half inside a jewel shard/death god where they are still fighting to this day.
Or if you want to bet on the long shot....it might be something completely original. Given this series, I wouldn't hold my breath on that.