Just some things so you can ponder the what if. Things that are still planned to be used in the future are going to be marked with spoiler tags. I'm going to warn you, the spoilers are going to spoil plot points in the next major arc after Antium and the Invasion is resolved.
Garnet, the Gun Mage
Riley was originally supposed to bring whatever he had on him and his body would turn into blood to merge with Garnet's. As a result, everything Riley had on him was left behind and Garnet would take those items with her. One of these would be Riley's Gun, from which Riley would use to cast all the powerful spells with the gun as a catalyst. The idea was ditched for just having her just go as a pure mage with Riley's body left in the other world. This somehow led to the basis for Blood Magic.
Charles Wellington, The Worst of the Worst
Originally, he was meant to be a bad guy. In fact, the worst of the four nobles of Antium. One of the things he enjoyed was strangling Garnet while sexually assaulting her and Garnet uses her strength and this opportunity to reverse rape him and force him to fight the other Dukes to death, creating conditions for a revolution in Antium that allows her to decide who the winner would be. This was too squicky and I decided to change him to be the one good who opposed the bad while the other two remained neutral. The horribleness of his character was divided into King Caligula, Boelyn Gray, and Baron Gray. Just think about that for a second.
Garnet's Sparring with the Guards
Garnet was originally supposed to make it to the sparring grounds with the other guards after Duchess Kiko's main guard, Camilla, offers to train Garnet. This duty was eventually passed on to Anna, one of Charles Wellington's guards.
Fleeing Antium
Originally, Garnet was supposed to run away by herself to Anzio when she learns that Zedd is lying and Charles Wellington was not there to help her. So, she fled north on her own, making her way to Anzio by herself.
Raid of the Red Manor
Assassins sent out by Duke Gray to kill Duke Wellington make their way to Duke Wellington's residence. Ruby is killed and causes Garnet to start mentally deteriorating. She seeks out Samantha Freud in order to learn memory magic, which combines with her own and results in the creation of memory manipulation magic in order to seek vengeance against Duke Gray by literally mind raping members of his family. Again, I felt it was too much and decided against it.
A case of theft: this story is not rightfully on Amazon; if you spot it, report the violation.
The Previous Hero
Spoiler :
Name: Assad
He is the previous Hero that was present during The Great War of Constantinople. In a last ditch attempt to halt the Invasion, he flies into the eye in the sky, causing the eye to disappear and the Invasions stop for thirty years before resuming. It is believed he died, but in reality, he has not. He has ascended from Hero to Overseer in an attempt to stop the ones who are Invading other worlds. Thus, a "Hero" no longer exists in their world. The Hero System itself was created by the Yellow Emperor in an attempt to find others who had this potential. Assad is the only one who has done it through the Hero System so far, making the Hero System itself largely a failure.
There are also people who falsely claim the title of Hero. Sometimes it is done to unite the people. Sometimes it is just because someone is being an asshole.
Anna
Spoiler :
Title: Embodiment of Metal
Anna is one of the five human form golems of the previous Hero, Assad. This is why she seems to not age and Garnet cannot see all of her stats. For example, she has no visible level because her level is based mostly on the strength of her creator. If I had to give her a number though, she would be above Zedd, probably level 120 or so. She is a Metal Elemental type and is capable of creating weapons without the need of spirits or Alchemy.
She is also responsible for creating the Space-Time Magic that Zedd uses. Anna created it in a desperate attempt to try to find Assad, but gives up on it when she realizes that her attempts and emotions could make her the next person to start invading other worlds, which is what she was created to help stop.
She is one of two known survivors of Constantinople. The other remains as a guard for the treasures left behind in Constantinople to ensure that its knowledge can be preserved and used to fight future Invasions.