This is really a very simple mechanic, so before I get to the how, let's start with the why. The above image is an AMD version of [#023 - The Long Road]. The premise is that it is a turn based, side scrolling beat-em up. Unlike [#023 - The Long Road], however, there are only two planes (instead of five) in this example. Ideally, I'd like to lower it to even a single plane - meaning that the player's character will only ever engage, at most, two enemies (front and back) at a time.
This reduces the tactical complexity of the playing field to the point where the best course of action is always obvious, available, and unsurprising. If you've ever played the first Dragon Quest game, which is the only JRPG that I'm aware of in which you only fight one enemy at a time in random battles, you understand how this simplicity makes for a rather unexciting experience.
Some games combat tactically obvious scenarios by limiting one's ability to take the best action every turn. For instance, in roguelikes, its not uncommon for particularly powerful items to be limited use. In collectible card games, the randomness of acquiring cards and them being one use means that you have to constantly adjust to an ever changing situation. Metal Gear Acid (and its infinitely superior sequel) manage to combine collectible card games with a tactical experience. As such, the game has great replayability due to how completely different the experience is simply by altering your deck a bit.
I'm taking a page out of Acid's book and providing something which is both a collectible card game and a tactical experience. Cards represent specific action, but may also be discarded to perform basic actions, like walking or attacking. The rules are as follows:
You have a deck of at least 30 cards, from which you draw two new cards into hand each turn. You can have up to seven cards in your hand between turns. If you have too many cards at the end of your turn, you must discard down to seven before your turn is over.
Each card represents a single action or ability when played. You have two actions per turn. You may choose to wait instead of playing a card.
Instead of playing a card, you may discard it to perform a basic action. A basic action is a basic attack (weapon damage + strength) or a basic movement (one square). You may discard up to three cards to increase the damage or movement, but you will still only get two new cards per turn.
When your deck can no longer supply cards, you lose your turn as your discard pile is reshuffled into your deck. You keep the cards in your hard through this process.
Not particularly complicated and fairly similar to Acid's approach. I threw in the ability to do basic attacks and boost your attack/movement through more discards to avoid those situations in Acid where you could fight back while you waited multiple turns for a weapon to be drawn.
The twist here is that you do not decide what cards are in your deck directly. Instead, each equippable item comes with specific cards that are added to your deck on your behalf. Some cards are only available in certain circumstances (like wearing the Happy Suit with the Happy Shield, or being a Warrior while equipping the Great Axe of Happy). Roughly half your deck is decided for you based on your class and equipment. The other half is your own "inventory" deck.
Weapons tend to come with cards that activate some sort of special attack with that weapon, though depending on the weapon, it could be many different things. A Fire Rod, for example, may come with a bunch of fire spells as opposed to the ice spells that come with the Frost Rod. The Cleric's Cane might make healing spells presentable, while the Thieves' Dagger may have cards for backstabbing, throwing the dagger, or even doing a dodge and roll action. Weapons also have a base attack stat that is added to your strength during basic attacks.
In addition to your main weapon, you can also equip something in your off-hand. For a shield, you'll get a defense boost along with some cards that will allow you to block against incoming attacks. For dual wielding, you would equip an off-hand weapon in the slot. These weapons have special attacks that become available when the weapon is dual wielded. For magic-based characters, you can hold a spellbook, which will make spells available above and beyond what your class and staffs provide, allowing a little greater customization.
Armor comes in three varieties (light, medium, heavy) and the cards that these armors tend to come with involve mobility and weight. For instance, light armor might have a card that performs a jumping action while heavy armor could come with stomp attacks or pushes. In addition, the armors are specialized, such that equipping the thief light armor would allow access to thief-like abilities and attacks, while the protector heavy armor comes with self healing abilities and damage resist buffs.
Your class deck is a group of cards that are defined by which archtype you select on character creation. These cards are made available as you go up levels, not unlike traditional class abilities. Though some of these are unique, not all of them are. For example, a knight can use some thief abilities, should he equip the tools that come with thief cards. The class deck essentially ensures that certain cards are available regardless of equipment. Your choice of class may also unlock additional cards on certain items. And there are some class-unique cards at high levels.
The rest of your deck is decided by your inventory. Similar to equipment, each item has several cards associated with it. However, unlike equipment, you can choose which of these cards to add to your deck. For instance, a potion might have a card for drinking it and another for throwing it at an enemy. You can have neither, one, or both cards in your inventory deck (inventory does not diminish - if you drink a potion, you can still throw it, and after you cards reshuffle, you can perform both actions again - think of it as a stack of potions rather than a single use-it-and-it-is-gone potion).
You can have as many cards or as few cards in your deck as you'd like. Your equipment is guaranteed to have X number of cards in it, even if those cards are just simple attack or movement abilities. Your inventory deck is dependent on what you are carrying in your inventory, but you are not obligated to add any cards to it. If you have an item, like a potion stack, that you sell, the cards are simply removed from your inventory deck. When you add a new inventory item, you decide then which cards to add.
This all happens between levels or in towns. You are stuck with one deck readout for an entire dungeon floor. Any inventory items you pick up during a stage are not available for use until you move to the next floor. Plan accordingly.
Your companions operate in a similar manner to the player, gameplay-wise. However, their deck is set and cannot be changed. In addition, they have fewer cards, both in their deck and in their hand each turn. For instance, a mage bodyguard may only have a dozen cards in his deck and can have no more than three cards in his hand at a time. Companions are meant to support the player, but not intended to be customized or to replace him.