Magni has so little written about him that you would think that there would not even be enough to fill even my short blog article. But this is not true at all ! From the story of the duel between Thor and Hrungnir we can ascertain a lot about his youth, and surmise much more about his future adult life. From a few lines we can build a short history for the God who will replace Thor as one of the guardians of Asgard and protector of Midgard.
Magni we are told is immensely strong. From the story of Thor's duel with Hrungnir: " Then Thjalfi went up to Thor to lift Hrungnir's leg off him, but he could not move it at all. When they heard that Thor was down, all the Aesir went up to him to lift off the leg, but they too were unable to do anything. After that Magni, the son of Thor and Jarnsaxa, came up to them, he was three years old then, and he flung Hrungnir's leg off of Thor". Now if every God in Asgard could not lift Hrungnir's leg, but Magni, at three years old could do it easily, then we know that he is by far the strongest in Asgard. What he will become as an adult can only be imagined by us, but I am sure it will make him the strongest being in all the nine worlds. Thor was so grateful he gave Magni the horse named Gold - Mane, which was Hrungnir's. Odin however, reprimanded Thor saying he was doing him wrong by giving the horse to the son of a giantess.
It is said also in these stanzas that Magni made the comment to Thor, after freeing him, " It was a pity I did not come sooner father, I reckon I would have struck the giant dead with my bare fist if I had met him". This shows us that even at the tender age of three Magni has a supreme confidence that is usually only shown by adults. I do not hear a cocky tone in this statement either. He simply knows his abilities and is very self - assured in his actions.
In several stories in the Eddas it is told that Magni and Modi, will inherit their father's hammer, Mjolinir. I believe firmly, call it a UPG if you want, that Modi will take up the belt of strength, the gloves, and the hammer. Magni will have no need of such implements in battle. Magni's raw powerful strength is more than enough to battle all comers and become victorious. With his vast strength and his bare hands he is a force to be feared by, and avoided by, all foes ! Magni's symbol, in my opinion, should be a clenched fist held aloft. A warning to all those who oppose his will, or try to attack Asgard or Midgard. Magni, with Modi, and the sons of Odin, will be the shield, and the weapons that guard the new Asgard that rises from the ashes of Ragnarok !
Go with Odin's wisdom, Freyja's love and Thor's protection !