Ok, so there have been this theory going around that the Spriggan 12 Neinhart is none other than Simon all because Erza and Kagura are going to face him at the port area.
In my honest thought, it is completely weird for Simon to be Neinhart. There are a few reasons why Neinhart should not be Simon.
1. It would totally ruin Erza's progression in TOH Arc.
2. I am certain that Altaris attack from the possessed Jellal definitely killed him.
3. Simon's body fell down a enormously tall tower will no way to break his fall where he would most likely have been skewered by sharp Lacrima pieces or most likely been crushed by large Larcima boulders.
4. When the Tower was shot up into the sky he would have most likely gone with it, if not then he would have been resting at the bottom of the sea with no effort looking for him because Erza and Natsu believe him to be dead and is properly crab poop by now.
5. If they did find his corpse how could they have brought him back to life? The only one capable of that is Zeref and he's on Tenrou Island at that time and that would leave August but if had that power he could have revived God Serena.
6. Before Simon died he had only one good eye, in Neinhart's silhouette he had two good eyes.
7. He has no strength that could possibly match that of any of the Spriggan 12.
8. Unless they want to make a shot-for-shot remake of Captain America: the Winter Soldier.