Balance Shifts
(Other suggestions for the metaname are welcome)
(First of all, my apologies for my poor English)
Premise: The conditions of the battefield change in direct response to the health percentages and momentum of each player.
Explanation:
Each battle begins with neutral conditions (no weather, no terrain, no room effects). Every 5 turns, OR when a player's total team health percentage drops below a specific threshold, the battle conditions shift. The type of shift depends on who has the lower team health percentage at the time of the shift. Per team, only one weather, one terrain and one room effect can be selected. Each player changes the name of three of their six Pokémon to select the 'comeback' conditions of their choice. The selected conditions apply even if the nicknamed Pokémon has fainted.
At the end of turn 5, 10, 15... [x + 5] (where x is the most recent turn the battle conditions got changed) the battle conditions change to what the player who has the lowest team health percentage selected. However, if the total team health percentage of either player drops below 75%, 50%, 25% and 10%, the battle conditions change to what the player whose team hit that team health percentage threshold selected.
Important: each total team health percentage threshold can only be
triggerd once by either player. For example: if the total team health percentage of player X hits the 75% threshold, the battle conditions change. However, if the player Y later also hits the 75% threshold, the battle conditions do not change. If player Y would hit the 50% threshold first, the battle conditions do change, while they do not change if player X hits the 50% threshold.
If possible, there should definitely be a feature that allows both players to see the total team health percentage of both team at all times to encourage strategizing.
Example:

(Excadrill nicknamed 'Rain Dance')
(Raging Bolt nicknamed 'Electric Terrain')
(Latios nicknamed 'Wonder Room')
Because of the nicknaming, the selected battle conditions are known to each player before the start of the match.
If the player with this team has the lowest team health percentage after 5, 10, 15... turns, OR their total team health percentage drops below 75%, 50%, 25% or 10%, the battle conditions change to Rain, Electric Terrain and Magic Room for 5 turns. If this player still has a lower total team health percentage than the opponent, the battle conditions remain unchanged.
Questions:
Q1 - A simpler variation to this idea, would be to only change the battle conditions every 5 turns based on the player who has the lower team health percentage. What do you think of this idea?
Bans:
Pokémon
At this point, I find it too hard to say what mons will be broken.
Abilities
All weather-setting abilities: Drizzle, Drought, Sand Stream, Snow Warning and Deltra Stream
All terrain-setting abilities: Grassy Surge, Electric Surge, Psychic Surge and Misty Surge
(F for no dimension setting abilities lol)
If it is possible to modify these abilities to not affect the current field conditions, these bans would not be needed. In addition, a pop-up could be given if a player searches for a match with a team that has a mon with a weather-setting ability (same pop-up when using banned mons/moves).
Moves
All weather-setting moves: Rain Dance, Sunny Day, Sandstorm, Snowscape and Chilly Reception
All terrain-setting moves: Grassy Terrain, Electric Terrain, Psychic Terrain and Misty Terrain
All dimension-setting moves: Trick Room, Wonder Room, Magic Room and Gravity
If it is possible to modify these moves to not affect the current field conditions, these bans would not be needed. In addition, a pop-up could be given if a player searches for a match with a team that has a mon with a weather-setting ability (same pop-up when using banned mons/moves).
Nicknaming
The only legal battle conditions to nickname your Pokémon are 'Rain Dance', 'Sunny Day', 'Sandstorm', 'Snowscape', 'Grassy Surge', 'Electric Surge', 'Psychic Surge', 'Misty Surge', 'Trick Room', 'Magic Room', 'Wonder Room' and 'Gravity'.
Steelroller - as it becomes a free 130 BP Steel-type move when any terrain is active.
Ice Spinner - as it clears the field of terrain. This is not needed if the secondary effect of Ice Spinner can be nullified with mods.
Other:
I am unaware whether some of these ideas would be impossible to implement in the simulator, so I would love to hear if there are such limitations.
Final:
As stated in my previous suggestions, I've always found field conditions an insanely unique mechanic added in mons. With this metagame, I hope to inspire people to make interesting teams that can function under the more obscure conditions of Magic Room, Wonder Room and Gravity.
I am once again very curious about your opinions on this OM. I am also very interested in hearing suggestions to improve, or even ideas you came up with to use in this metagame!