This is a cross between my other comp-grid combat ideas and a puzzle game, like Tetris. To put it simply, two sides are fighting along a line in the middle. The units on either side of this line are the front line. The units in the next column are support units. The rest of the units are inactive until the advance to the support/front line.
Units come in formations. You can think of a formation like a Tetris piece. Reinforcements are added to the field by "dropping" them in a particular row. They will then march up towards the front, going as far as they can while still staying in formation.
As combat at the front line goes on, one side may gain the advantage and destroy the opponent's column. When this happens, the line of scrimmage moves towards the opponent's goal (meaning the defeated side loses space), and the victor's units march forward to meet the new line. Eventually, one side is forced to drop in their commander. When this unit is defeated, that side loses the battle.
That's basically it, but it gets more fun in the details.
Let's start with the basic concept. All units in the game belong to a formation, which like I said, resembles a Tetris piece (though they can be as small as 2 units or much larger than 4). Formations move as a single unit. If they can move forward in formation, they will. If they can not, they will stop. Again, like Tetris pieces.
The next reinforcement is chosen randomly, though if you wanted to go the extra mile, they could be selected randomly from a prepared deck of chosen units to allow for some customization. You can see the next couple formations in the queue, and perhaps modify the queue through some powers. You do not have to send in the reinforcements immediately - or at all. You can just sit on them as long as you like. Of course, if your side of the field is empty, there's nothing preventing your opponent from capturing the whole field.
Reinforcements take time to appear after being used, during which time they will block those rows from further reinforcements. Think of it like holding a Connect Four piece above the hole. You can still put pieces in the other slots, but you can't use that slot until you release the piece. This is to encourage the players to spread their units out, and to slow down the rate at which reinforcements appear.
The commander unit is very powerful relative to other units, but when it is defeated, the battle has been lost. The commander's formation will appear after X number of reinforcements have been used. The player who uses primarily quick spawning reinforcements will have more units on the field sooner, but will end up having to field his commander sooner. The commander is not the final unit sent. You can continue sending in reinforcements, but he'll be at the front line sooner or later.
Combat at the front line is simple and obvious. Units in the front column use melee attacks against one of their three adjacent enemies. Units in the support line can use ranged attacks, magic, healing, and stuff like that. When an enemy unit is destroyed, the reinforcements will move into to take his place, when physically possible.
When a formation loses a unit, it just drops that square from the formation. This could lead to the creation of two or more smaller formations if the unit lost was in the middle.
Units do no immediately die. Instead, once their hit points are completely gone, they enter a "wounded" state. While wounded, they attack at a greatly reduced rate and do less damage when they do hit. They can take no further damage, but after 5 seconds, they will die and be removed from the board.
In some cases, one side gains a significant advantage over the other and is able to gain ground, moving the line of scrimmage. Basically, whenever the opponent's line is either empty or filled with wounded units, that line is destroyed and the front line advances. Gain ground has a short timer of a few seconds, so even if the opponent's field is completely empty, it will take time to completely conquer it (giving him a chance to drop some defensive reinforcements).
Gaining ground is important for a few reasons. First, if your opponent can not field any more units because you've taken over his field, you win, even without destroying his commander. Also, less field in general means it is more difficult to set up reinforcements since you can't stack as many.
Second, the number of reinforcements before the commander comes up are set for each battle, but each time you gain ground, you effectively steal a reinforcement, gaining an additional one for yourself and your opponent loses one. If he manages to gain his ground back, he'll steal the reinforcements back. Long story short, gaining ground makes the enemy commander come out earlier.
If a column is completely filled with units (no holes in the line), they get a small defensive bonus to the entire column. There are other small bonuses that a column can get based on the units in the column. You may have noticed that the units have colors. These don't affect the unit directly, but instead are used to figure out the line bonuses.
For example, the line may have a bonus to damage done if there are three red consecutive red units in a column. This bonus is increased by more consecutive red units or by having a second run. So a line that is all red will have a significant damage boost. Having no runs at all (alternating colors every unit) may also provide a different kind of bonus.
Finally, as is typical of my comp-grid entries, the units themselves can affect the units the touch. For instance, an animal tamer may increase the power of any wild animal-based unit touching it. Putting a cleric formation behind a warrior set could heal the warriors once wounded. Put heavy armor units in the front line with archers in support behind them. And so on.
Large units are a single unit which occupy more than one square, like a 2x2 Ogre. These units work just like smaller ones, except that their demise will result is many more squares being vacated.
Finally, there's what I call the Voltron Effect. Some units will merge with similar units to produce larger, more powerful groups. For instance, if you drop a 1x2 set of archers onto a set of 2x2 archers, they may merge and become a set of 3x2 archers. In general, these groups gain a little more power at the cost of larger formations.
However, certain types of units may produce a completely different unit in their place. If you stack three 1x2 units of cultists, they may spawn a 3x2 Cthulhu where they otherwise would've have been. Just a little bit of Facetris in there, just for fun.