WARNING: SPOILERS IN THIS POST FOR BOTH GF4 AND GF5. ALSO VERY LENGTHY. YOU HAVE BEEN WARNED.
I've managed to add new characters into the game. You're right alhoon, the Zone Simulator isn't too terribly difficult to use regarding adding characters; certainly not as bad as I was expecting it to be. Which is nice.
Anyway, what I've done first is add Shaila into the game. You can get her in Isenwood's Spire, and only if you turn in Mehken to Rawal (Most players seem to do that anyway; Mehken as a companion is short lived, and the extra stats are nice. Might change this later though) (Note, most of this is for experimentation only; no mod releases from me yet. This is me rambling my thoughts and what I'm doing. Any modders reading this may get some ideas though).
Why did I implement her? Well, I was a little disappointed that we hear utterly nothing about her after GF4. True, she is a minor character, meant to be your early game boss in GF4, and easily gotten rid of and forgotten by the player, but on the other hand, she was able to get the attention of both the rebels and Shapers, which isn't something to scoff at. My speculated back story for her is that she might've been an apprentice mage trying to learn more potent magic, but the Shapers, for some reason or another, make it very difficult for her to learn more magic, thus preventing her from becoming more powerful (think that mage in Fort Vengeance, if you've gotten that far in GF5. I think her name is Cyprien, or something). It's one of the few explanations I can think of that would explain why she would turn to the rebels in the first place.
My thoughts are that during the events of GF4, she was brought to the Foundry, where the GF5 Protagonist starts. Sorta thought that since the Foundry is the place that studies the modifications caused by Shaping (The Foundry seems to analyze any anomaly that comes from Shaping itself), they might know what to do to fix what the Geneforge did to her (Given that she was essentially Shaped by the Geneforge). Of course, given that Rawal runs the Foundry, he implants a control tool into her and pretty much does nothing to cure her (I wouldn't really expect a lying rat like Rawal to keep the Shaper's promise to cure Shaila).
She is cured the same way the GF5 protagonist is; using the Geneforge that Rawal has been keeping a secret. The reason is because the GF5 protagonist, before using Rawal's Geneforge, is shown to have been heavily damaged, mentally and physically, from too many modifications that were made too quickly or sloppily. We know that canisters can cause mental instability, and that the Geneforge is even more severe in this regard. We also know (in the case of poor Shaila) that the Geneforge can cause serious physical defects in individuals. How using a Geneforge twice fixes mental instability and physical problems from over-modification is beyond me (since using a Geneforge twice in GF4 kills you right then and there...), but hey, it works out for Shaila in the end, since she is cured ,ironically, by the same device that got her into this mess in the first place.
What I was hoping to do was something like your companions in GF3. Weapon upgrades, new abilities, upgraded resistances, companion opinions, etc. The weapon upgrades in GF3 were likely hardcoded, meaning that they can't be replicated in GF5 exactly like in GF3, but there is a workaround. Using the Zone Simulator, you can just add a new character to a zone, essentially your companion with all of the upgrades you want, then make it so that once you pay certain NPCs to train your companion, it simply takes the current companion out of your party and replaces it with the new, upgraded one. It's rough code wise, since you do want to make sure you don't have duplicates of your companions for example, but it does give you more freedom, because you can change the graphics, resistances, attacks, etc. of your companion much more.
Shaila as a character I imagine would be very adept at using debuffing magic, causing debilitating status effects (such as curse, weaken, fear, charm, etc.) on her enemies. Stuff like Daze and Fear seem to be essentially messing with the opponent's mental state directly, and given that her mind was completely warped by the Geneforge, she probably pretty good at screwing with the enemies' heads. I wish there was a way for attacks to do multiple status effects on enemies, but I have not found any way to do so yet. So for now, this is how I mapped it out for Shaila in terms of power:
EARLY GAME (Dealing with the Presence, doing stuff in the Whitespires, etc.): Shaila is the same as she was in GF4, stat and ability wise. I literally took her stats in GF4 and copy pasted it into GF5. She does have quite a bit of base health (150, to be precise), but her Ice Spray attack isn't going to be the most impressive thing in the world.
POST-GENEFORGE USE (You have defeated the Presence at this point, essentially.): Because she is restored by the Geneforge, Shaila is going to be stronger than before, given that now she can channel her power without being distracted by the physical pain and mental instability plaguing her before. Her Ice Spray now will be equal in power to the regular Ice Spray that the PC gets from NPC trainers. She also gains some small stat boosts. Not sure if this is too much in the early game, but I'll take a look when the time comes.
MID-GAME (Mera-Tev, helping Astoria): With a fee, Shaila can be taught stronger magic by Shaper Quothe. After her training, she uses a slightly less energy intensive (but no less powerful to the regular) version of the Essence Orbs spell, as well as a few bonuses to stats and resistances. She gets some resistance to mental effects, such as charm, fear, and daze. Additionally, her melee is capable of inflicting the fear status effect in enemies that get hit by said melee. You're not going to use her melee much, given that she's not supposed to be up at the front soaking up damage, but it's decent for 'oh sh*t' situations where enemies bum rush her. Gives her a little bit of crowd control and some time for you to take out enemies that aren't running around in fear. Might be a little too much in this part of the game, but I'll see.
LATE TO VERY LATE GAME (You're in the Storm Plains, Dera Reaches, Gazaki-Uss, getting Alwan's quest, meeting Ghaldring, meeting Taygen, etc.): Shaila gets her last and strongest upgrade at this point from Sage Pavyl in Stormhold. It will take quite a bit of money to get the upgrade, but it is one of the strongest upgrades. She gets large bonuses to stats and resistances, as well as a less energy intense version of Dampening Field (The AoE Curse attack Podlings use against you) that does significantly more damage compared to the ones that Podlings use. Her melee now can Slow+Fear (found a limited workaround so it is possible to have two status effects on a melee attack), and she becomes completely immune to mental effects, such as fear, charm, daze, etc.
What I want to know if its possible to have attacks (both ranged and melee) have multiple status effects inflicted upon an enemy (such as curse+slow, or fear+curse, etc). I'm also curious if it is possible to have zones be changed in terms of appearance over time (Something like the Shaper school in Greenwood Isle in GF3, where you leave the school, and when you return, there are more people and the areas have been cleaned and swept, to show that the school is being rebuilt). Also, thinking about it, the Zone Simulator can also be good for modifying zones that have disappointing boss fights (IMO, the fight with Greta in Inner Gazaki-Uss for example is pretty underwhelming; she is, code wise, literally a renamed generic agent, nothing else. It would be pretty easy to replace her code to have a unique character for her. I found her GF4 incarnation, the one where she uses Essence Orbs and has a stronger melee attack, to be much cooler and stronger. I could probably think of some other things for her too while I'm at it, but that's for later), or if modders want to make certain zones even harder to get through (probably wouldn't be hard to add a ton of enemies in certain zones, or make enemies stronger, or whatever else is possible).