I've mentioned it before, but I'm mentioning it again because I've made some head-way in implementing it, the Titan Coolant system. Titans have a variety of systems that don't directly affect combat, but can be used to improve their combat capabilities, including crew and capacitors. I want to add a coolant system that enables players to draw activated cards back to their hands. This needs to work with other systems like crew and capacitors, and not be an overwhelming no-brainer.
Currently crew members can add their cogs to the cogs of whatever system is subsequently activated. If they're re-activated, they can be used twice. So the cost of a coolant system at charge 1 seems pretty reasonable as it will return an activated card to your hand at the cost of two other cards, the coolant system and whatever is paid to activate it. Using a plasma shotgun or howitzer twice in the same turn would require the player have at least seven cards in their hand, three for the first shot, two to recover the plasma weapon, and two more for the second shot.
Maybe the player has a capacitor in their hand, then the player can attack with the plasma weapon for three cards, recover the cost using the capacitor, recover the plasma howitzer using the coolant system, for six cards. Time-wise, though, that's four turns compared to three for not using the capacitor. So pretty reasonable in terms of combos, although one might wonder whether that is true if a junior crew member is used to recover both plasma weapons on the turn they've been used (requiring nine cards, or eight including a capacitor) or one plasma weapon and one master crew member. That's a total of three cards and two turns to recover those.
It's also possible a player might recover those cards in time for the next round, meaning the player only needs to build up eight cards in their hand. There's the possibility of a Titan having two coolant systems and going for a kind of perpetual motion machine using one to reactivate the other and a crew member, who in turn reactivate the first and a crew member. Keeping the charge cost at 1 ensures that doing this will have diminishing returns, although a caveat making sure that players don't attempt to reactivate the coolant system being activated will probably be required.
Finally, I want the reaction to be something interesting and different from the three other reactions available to the players, and perhaps to port that reaction to the jump jet and thruster systems: things exploding to the detriment of the attacker. In particular there is a scene from Pacific Rim whereby Raleigh opens up the Gypsy Danger's coolant system (to no apparent detriment) in order to freeze Otachi's tail while it has been blocked by Gypsy Danger's arm. How to accomplish something like this in Titanomachina might be to cause the attacking system to take damage equal to the damage it is dealing out to the target, or to take only one point of damage, or some middle ground that doesn't make Titans with coolant systems invincible in close quarters. Figuring out how that would affect weapons like plasma howitzers or macro lasers would certainly be an act of imagination. Mind you, super-cooled weapons attempting to discharge are probably wildly unsafe. Maybe it would be something to say that systems being activated to attack from an adjacent square suffer damage equal to their cogs (ignoring any crew bonus or damage penalty) when this reaction is played.
Edit: Another two ideas, the second of which I prefer:
1. One of the systems on the reacting Titan is deactivated, to be put back into the player's hand. This is a more powerful version of the card's basic action. It prevents the card from being pulled back and activated again that round, but most of the time the end result is the same for the cost of one less card.
2. It deactivates an attacking system at range 1 unless the attacking player pays an extra card face-down. Which is really specific, except much of the game tends to be spent at range 1 and the result matches that of the Gypsy Danger vs Otachi frozen tail thing.
