There are many ways in which Pokemon Legends Z-A is different from the mainline Pokemon games, which will be discussed below.

Pokemon Legends Z-A is the newest game in the Legends series, which has already shaken the formula from the mainline games. However, there are many further changes that this game will have.
Of course, many things remain the same, like held items or catching Pokemon through battling. However, there are many changes to the core mechanics of the game, all of which will be seen below.
How Is Pokemon Legends Z-A Different From the Mainline Games

There are multiple differences between the predecessors and this upcoming game. Some of these are obvious, while others need an in-depth look to be noticed. These are:
Battle System
This is one of the foremost and most obvious changes you will notice within the game. The turn-based style, which Pokemon is known for, has been thrown away. Moreover, the battles are more dynamic in nature. You have to manually input the movements for your character, while your Pokemon follows you.
The moves cannot be spammed and can instead be used after a cooldown. Abilities are not present in the game (although this was not there in Legends Arceus as well). These are some of the changes that are just the tip of the iceberg when talking about the battle system for Pokemon Legends Z-A.
Apart from these, the moves also work differently. Lots of moves have different animations and a differing area of effect. This will make some moves easier to land over others.
Mega Evolution

Traditionally, you could use Mega Evolution whenever you want, as long as you had the Mega Stone for the respective Pokemon. However, this is going to be revamped, at least in some cases.
The demo footage has revealed the changes this mechanic has undergone. Now, when you fight against Rogue Mega Evolution Pokemon, you will have to use your moves and collect the energy that gets released from the collision of your move. Once you have collected enough energy, you will be able to Mega Evolve your Pokemon for a short period.
This is another change that the system has undergone. Earlier, Mega Evolution would last until the battle ended. However, in these battles, it is a temporary power-up, which goes away after a while.
Starters

Every region has its own set of starters. For Kalos, these are Chespin, Fennekin, and Froakie. However, even though Legends Z-A takes place in the Kalos region, the starters are not the same as before. Instead, this time’s starters are Totodile, Chikorita, and Tepig.
This is not completely unexpected, since Legends Arceus also used previously known starter Pokemon, albeit with regional forms. However, the above-mentioned Pokemon do not seem to have any indicated regional forms till now.
Story Progression

In the main series of Pokemon, you progress the game by battling various gym leaders. However, Pokemon Legends Z-A is putting a stop to that, since the game is set within Lumiose City. Instead, the likely method of progressing the story will be through the Z-A ranking battles.
This is also an upgrade over Legends Arceus, where you had to fill the Pokedex entries to progress to the next region.
Setting

Every Pokemon game in the mainline series has let you explore whole regions (and sometimes even more, in the case of Generation 2 and 9). Even Legends Arceus lets you explore the region of Hisui. Meanwhile, Pokemon Legends Z-A is dialing back the scope of the game by limiting you to a single city.
The entire game will likely be contained within the limits of Lumiose City. That will wrap up how Pokemon Legends Z-A is different from the mainline games.

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