So I've been reworking my idea for a battle mechanic for a "fan game", thoughts?
PokeSouls
Basically, this is kind of like "summons" or "spells" that the Pokemon Trainer themselves kind of use. Effectively, it gives you access to up to six additional moves per battle. The Pokemon still use the moves themselves, but the Trainer has their own set of PokeSouls for each fight that any of their Pokemon can utilize.
There are 54 PokeSouls. They're divided into the 18 Types, and then divided down further into ranks---1-Star, 2-Star, and 3-Star. Using the "Soul Driver" Device, you can equip per battle up to six-stars worth. So, six 1-Stars, three 2-Stars, two 3-Stars, or a combination. Each PokeSoul grants access to a basic Elemental Type attack that does either 40, 60, or 80 damage. You select a Physical or Special move in the Pokemon's arsenal to determine the Damage Category, but unlike Z-Moves nothing else about the base move matters. This is known as a "Soul Burst", and once per turn any Pokemon on your field can utilize. Effectively, this gives your entire team extra coverage. Here's more details:
Soul Burst
Once per turn, a player with a non-sleep/non-frozen Pokemon can activate a Soul Burst. By choosing one of their PokeSouls, this allows the Pokemon to utilize the corresponding Soul Burst.
A Soul Burst's Type is based on the Type of PokeSoul, which means that by using a Soul Burst a Pokemon can potentially use ANY Type of attack. If the Pokemon is of the same Type as the PokeSoul, they'll gain STAB, though the point of Soul Burst is to typically allow Pokemon access to a Type of move they wouldn't know otherwise.
A Soul Burst's Base Power is based on the Rank of the PokeSoul. 1-Star is 40BP, 2-Star is 60BP, and 3-Star is 80BP. Soul Bursts ignore negative changes to the user's Attack or Special Attack. They also ignore Abilities and Items that activate in response to moves, be it the user using the move or the target being struck by a move. For example, if Dewpider uses a Water Soul Burst on Sandygast, Water Bubble wouldn't increase the power of the move, nor would Sandygast's Water Compaction activate. However, if Sandygast already had a Defense buff due to Water Compaction previously, the buff would still be taken into account. Electric Soul Burst wouldn't activate Lighting Rod as that's an Ability that "activates", but a Pokemon with Levitate would dodge Ground Soul Burst since that Ability is passive. Of course, this mechanic would probably need to be examined in terms of working with Abilities and Items on a case-by-case basis.
A Pokemon channeling a Soul Burst also must know a Physical or Special move. This is called the "Medium Move". The actual effect of the move doesn't matter, but the Physical/Special category must be chosen to determine if the Soul Burst is Physical or Special. So, if Tepig uses Electric Soul Burst, it could choose between Tackle and Ember for instance. Tackle would make it an Electric Physical move, while Ember would make it a Fire Special move. The damage and Type of the Soul Burst is the same regardless of what move is chosen, this just determines Physical or Special. A Medium Move must have atleast 1PP to be selected, but can be selected ignoring the effects of Disable, Cursed Body, Choice Band, Etc.
The turn order is decided as usual based on Speed. HOWEVER, if both players use a Soul Burst, a Soul Clash will occur. In this case, the user with the higher Star Rank will go first regardless of Speed. In case of the same Star Rank, Type advantage determines the order. So if Player 1 uses Fire, and Player 2 uses Water, Player 2 would go first. Immunities count as winning---so Ground beats out Electric---and in the case of a tie, like Dragon vs. Dragon, Speed is once again the factor. Normal is unique. Since Normal doesn't have any Type advantages, it will always win out in a Soul Clash unless up against Ghost. In that case, it's a tie as Ghost and Normal don't effect each other.
Soul Bursts ignore the accuracy check, and will only miss if the Pokemon is using Fly, Dig, or Dive and off the field. Protect/Detect/Etc. won't fully block the attack, only reduce the damage to 50%.
Mega Souls
Mega Pokemon are also incorporated into the Soul Driver, through Mega Souls. Now to Mega Evolve, you insert a Mega Soul into the Soul Driver in the center. You can only have one per battle, so choose wisely. You can Mega Evolve once per battle, HOWEVER once your Mega Evolve you lose access to the other PokeSouls. So do you wait and use the PokeSouls first, or Mega Evolve? Though Mega Pokemon don't need to hold Mega Stones, when they Mega Evolve they temporarily lose their Hold Item for balance, though before Mega Evolving they can freely make use of the Hold item and PokeSouls for additional strategies.
How I Imagine This In PVP
I'm not a competitive player, and I thought of this idea first-and-foremost as a single-player mechanic. But I think the idea of basically giving each player a "universal" set of spells all their Pokemon could use might be interesting.
The way I see it working in PVP, every player would have access to all the PokeSouls at the beginning of each battle. You'd see the Pokemon Teams, and then the players could construct their PokeSoul load-out to best counter the opponent---and then when the battle begins, you could see what Type of PokeSouls the other player has, and they could see yours, so it adds an extra layer of "do I use this now, or is he planning to use his" to the game---and of course I'd imagine it's a mechanic you could switch off since it isn't "baked in" to the Pokemon themselves.
And in 2-Out-Of-3 Matches, it'd be even cooler to then readjust the PokeSouls based on what moves you saw the opponent use to cover your weakpoints.