Assume for a moment that at Zidane's next birthday that celebrates another decade of life, after the events of the game, he starts bulking up. Garnet lives a long life and comes to her mortal end yet Zidane does not. After a mere hundred years, during which the nations of the former 'Mist' Continent regain their strength. So he reforms Alexandria's military - only to awaken a day after his task is mostly completed (another decade) to find the Emperor of Man standing outside restored Alexandria castle. (Assume Alexander was put back into a restored sword/tower by Garnet before her death. Also I do belive this is a Out of Context Problem for any nation of Gaia, a couple centuries of regrowth or not.)
The Emperor wants Zidane to found a legion, instead of whatever tragedy befell whomever was the canonical Primarch XI.
He agrees, using his already made forces to help theme his new forces. His legion is brought to the castle where he shall give a speech before his new troops. Assume he does not die of shock and takes the news of the galaxy as well as can be expected. He also easily wraps his head around the IoM's technology fairly easily. (Telepathic implantation of the knowledge a week before his meeting with the GEoM.)
What would Zidane's legion look like, and what other Space Marine Legion specific thematics can you come up with?