I don't expect anything to be perfectly balanced, but I always want to feel like I'm playing a legitimate game without obvious issues making combat trivial. I hate that I have to ban myself from using things to make the game fun. Some relatively small changes to the game would help a lot, see below:
Glass cannon: Really now, this talent should increase damage output, not AP! Why does glass cannon turn me into a bionic man that can suddenly run or swing a sword twice as fast? It doesn't make sense and breaks combat by allowing for non-stop kiting or massive barrages of CC. Glass cannon thematically suggests higher damage at the cost of defense, not super hasted bionic limbs
Invisibility spell: No amount of int should reduce this spell to a 1 turn cooldown. This is doubly true considering no game mechanic exists to detect an invisible character. You should enforce a minimum cooldown on game-breaking abilities such as this. For example, it could have no less than a 5 turn cooldown regardless of int.
Stealth: Again, because a high stealth character is near impossible to detect, and it only costs 1 AP, using stealth in combat needs a cooldown. You could simply make it so a character can't use stealth in any round he/she has already used an offensive action. This gives the enemy a chance to fight back in between sneak attacks, and it makes sense that your character would be temporarily spotted after attacking from the shadows.
Elemental shields: I can't believe these are still doubling our HP! Come on, they already give resistances and immunities for 3 AP, that is good enough. I shouldn't be able to double my HP every turn for 3 AP, how boring and overpowered!
Glass Cannon: Nice idea, actually. A simple damage multiplier might work. They'd have to check on how it would work with resistance lowers debuffs on enemies and damage-boosting buffs on allies, though. Imagine a Raged, Oath of Desecration, 2H fighter using Flurry on an enemy which has had its Resistances Lowered.
Invisibility: I'm not sure what's game-breaking about this? Is it just that you can steal things freely? I tend to use Invisibility as a defensive ability, and I think Invisibility needs a slight buff in one area to be honest - I think that taking damage from surfaces, or the burning and poisoned statuses should not break invisibility. I tend to use it on characters who are in deep trouble... but because it breaks from burning and poison and surfaces, it doesn't really help them survive.
Five turns is also 30 real-time seconds, and that could be a problem or at the very least an annoyance if you had to wait around for that in the stealth sections before and during the Luculla mines, and later the Demon path to the Giant Bloodstone in the Plains.
Stealth: This is tricky, because I definitely know where you're coming from. I basically killed the final boss almost entirely using a Guerrilla Shadowblade, the boss didn't react at all. I missed like three times, but that was all the damage I took. So I agree that it can be too powerful and enemies should react better to it.
On the other hand, though, if you make Stealth and/or the Guerrilla talent too restricted in when it can work, it'll end up being basically worthless. An unanticipated consequence of the stealth restriction could be that Rogues would have a hard time retreating from combat to hide and heal. Sometimes they can be close to enemies and need to blind/stun them to avoid being hit by an AoO. Under your restriction, wounded Rogues could no longer do that and then go hide to avoid being targeted.
So I think a better solution is to have enemies AI react better when backstabbed, as in looking around or moving, rather than just restricting Sneak like that.
Elemental Shields: I agree that they are quite imbalanced. They're available really early and are much, much better than the Immunity spells. They either shouldn't do as much as they do now or be available so early - probably both.
