You could add inter-character synergies through resonating crystal frequencies(yay!). In non-salesman words, you could give some bonuses when a skill chain of a given crystal type is executed.

For example, you have a warrior with a Slash skill. You augment it with a Fire crystal so it becomes Slash with a burn after effect. Whenever you hit an enemy with this skill, it stacks 1 Fire element on the enemy. Call it residuum magic. It will either decay naturally or when using an opposite element. Your other character has a Fireball skill. Since the target has 1 Fire element, its damage is improved and the target now has 2 Fire elements. You could add finishing skills which consume all elements for a big effect depending on how many of them were stacked. This is a mix between FF13 stagger gauge and WoW rogue mechanics.

You could also expand it by allowing residuum to work offensively and defensively. If you got Fire residuum on your allies, casting a Heal skill improved by Fire could consume them and have some effect(ex: heal over time), effectively turning the enemy's strength into your strength. Similarly, you could self buff with Fire residuum to provide Ice resistance. Maybe casting a Fire skill gives you 1 Fire residuum, so it then becomes a 2 edged sword. Against Ice enemies, it dominates since Ice is weakened. Against Fire enemies, they enjoy increased damage.

The idea is to mimic deck building from Magic. Using similar elements creates synergies at the cost of flexibility. You can choose to focus on a single strategy and be very good at it or be weaker but be prepared for more situations. That would add a bit more depth to skill customization and some interesting combat dynamics. However, for this to work you need to have a limited number of active skills per character. You don't want the player to run around with similar skills covering all elements or you end up losing the strategic portion of the system.

The first idea is basically Chrono Cross' battlefield system. I didn't actually like it in practice (though it looked good on paper). It just prevented more than it allowed. There was strategy involved, but most of the time, it was all about "pick one element, equip everyone with that and unload"

Adding an element signature to a skill is a good idea. It feels new to me, though I may obviously be wrong. Altering the skill effect substantially would also make sense.
For example:
Powder = healing
Powder + Fire = burning enemy
Powder + Wind = Sleep status
Powder + Earth/Water = Growth (Buffing self)


The residuum idea sounds ok, but it is restrictive to a single element whereas I don't see how earth should 'dispel' fire or 'wind' dispel 'water'. Besides, it is a bit too similar to how I envision status effects (imprints) thus this system would feel redundant in this specific project.

The idea is to mimic deck building from Magic. Using similar elements creates synergies at the cost of flexibility. You can choose to focus on a single strategy and be very good at it or be weaker but be prepared for more situations. That would add a bit more depth to skill customization and some interesting combat dynamics. However, for this to work you need to have a limited number of active skills per character. You don't want the player to run around with similar skills covering all elements or you end up losing the strategic portion of the system.
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Yes, I was thinking 5. I realize this is a bit restrictive, but 5 skills * 3 characters is still 15 tools you bring with you on the battlefield. Provided the implementation of the previous suggestion to skill-swap at a cost, (which increases flexibility at the cost of in-game resources) this is a lot to work with.

Tiblanc's Residuum is actually something fun to play with. I think what you want to do with the Residuum magic is to use it as a "resource" with simplified effects.

For example, each Residuum-Element stacked on the target increases the effectiveness of said element against the target. Conversely, it would reduce the effectiveness of skills that is weak against the stacked element.

Examples:
1) "Character A" is stacked with 3x Fire Residuum. All Fire-Based skills (both offensive and defensive) targeting "Character A" is 30% more effective. That means "Character A" takes more damage from Fire-Based attacks and gain larger amount of Fire-Based healing skills.

2) Same as the above, but "Character A" is targeted by Ice-Based skills. Ice-Based skills are 30% less effective against "Character A". That means "Character A" takes 30% less damage from Ice-Based attacks, but also receive 30% less healing from Ice-Based healing spells.

3) Lets say you have a Fire-Based skill called "Explosion". "Explosion" consumes all Fire Residuums stacked on target character, and deals [INT x Fire-Residuum] as Fire damage to target.

4) Or if you have a buff call "Fire Power" which consumes 1 Fire-Residuum on target and give target +1 STR until end of battle.

5) The "dispelling" part comes in to help clear the Residuum stacked on the characters. If "Character A" is attacked with Water-Based attack, the Fire Residuum is completely erased, and replaced with 1 Water Residuum.

That way, when you engage in battles, players must be careful on how they stack their Residuum on both their own character as well as their enemies. If they overstack a certain Residuum on the enemy, the enemy could turn it against the player by consuming it and using it as a buff. Similarly, players can also stack residuums on their own character (via buffs). Overstacking can make the character powerful, but in the same token the enemy could unleash a nasty spell that make use of the same residuum and blow up the character suddenly. It keeps tension in the air.

Also, you can make buffs and debuffs scale with the number of Residuum stacked on the target. (For example, Ice Shield: Increase target's armor by X, where X is equal to the number of Ice Residuum the target has). I think the Residuum has potential to make the game a bit more fun and chaotic while forcing players to be more strategic by constantly reconsidering his own situation before making any choices.

I feel like this would be a lot of information to keep track of on the UI which may have a not-so-appealing look :S
Also, isn't using fire residuum on your characters against a water boss kinda pointless? he will end up dispelling your guys every round won't he?

I feel like this would be a lot of information to keep track of on the UI which may have a not-so-appealing look :S
Also, isn't using fire residuum on your characters against a water boss kinda pointless? he will end up dispelling your guys every round won't he?

If Water > Fire, then yes. He'd have a more difficult time getting rid of lightning though, so I'm not sure why you would go in there with a fire build unless you're stubborn.

And yes, a good UI makes or breaks combat systems. It's important to be able to convey information efficiently to the player or the complexity will become unmanageable.

I feel like it would be dispelled too easily by other alignment-spells which makes me feel like this is not a viable system as it currently stands.
An alternative would be to only allow the player characters to control residuum (not enemies?)
Generally, I feel this adds a lot in terms of complexity and I'm not too sure I want to complexify my battle system by that much.
I'd rather have something that affects skills outside of battle for customization, but it rather 'invisible' once the battle start.
For example, a fire spell being attached which something that makes it an AoE fire spell and the likes.
Maybe that's a tad unoriginal but I feel that's the design space I have left to deal with.

How about creating combo skills like in Chrono Trigger? Have some skills create new combo skills when you augment them with a similar crystal. That would fit the invisible property you want and encourage character builds to align in a common direction.

Chrono Trigger was a very accessible game, but it was also too easy.
In my early design I had double and triple techs, but it occured to me they were rarely worth it.
In chrono trigger, I rarely found great use of the triple tech even if it was found from optional rocks and whatnot.
Too often, having the flexibility of 2 damage dealers and a healer was much more efficient than a single devastating spell.

Besides, I don't think synergy of characters needs to be encouraged. If it feels necessary, players will get there through trial 'n error by finding more and more efficient ways to deal with threats.

By analogy, this makes me think of the 'drop 9' in Magic the Gathering.
Competitively, no one really plays any drop 9 unless they can lower its cost significantly, and even so, it rarely sees competitive play.
The thing is you often get to win a game with 3 drop 3. This means you'd expect your drop 9 to win you the game altogether, with relatively no risk, but it is often too frail and vulnerable.
I feel like dedicating 3 of your characters to perform one particular ability is committing a lot for nothing. It takes away 2 player decisions in the process (instead of doing a, b and c and hope that my synergistic strategy works, I do D and cross my fingers and blame it on the AI if I fail).

I'd really stick with skill properties altering their use. I feel like this is the one aspect of the skill system that needs a bit more love.

I guess you need to stick with the interactions within a single character. How about giving secondary bonuses based on which type of upgrades are installed? You got 5 skills, which means 5 upgrades. You could have 4 levels of synergy bonus per type.

For example, you have 3 skills with AoE enhancement. You get Level 2 AoE bonus which gives you +15% damage for these skills. Or you upgrade them with defensive stuff which gets you Auto-Protect at some point.

You can expand this by making some upgrades incompatible and causing a penalty. You can also add combination effects so that mixing different upgrades gives you something.

That should keep complexity out of the battle and fairly manageable while giving lots of customization.

I think what I'm getting at is that the skill being attached to an enhancement is enough in of itself, but I just don't see exactly what to make of these attachments to make them stand out.
Adding damage is meh, sure it works, but its still not a thrilling decision. Area of Effect is already a tad better, but its not skill-changing very much.

The Auto-Protect idea sounds interesting, can you elaborate?