Seeing a lot of discussions around 3rd partying and especially solo teaming, I started thinking about ways to prevent or eliminate this problem.
The moment you start fighting with someone, both you and that player become invisible and immune to the other players, while the rest of the lobby also becomes invisible and immune to you. It could look like jumping into the void or time shifting (like in Titanfall 2). You and the attacker would stay in this "parallel dimension" until one of you is dead or escapes.
Eliminating the enemy would grant you about 5 seconds to loot the deathbox or heal quickly before getting back to the main dimension.
Escaping could be defined by a period of 20-30 seconds of hits not being registered, while opponents being invisible to each other or more than 250m apart. Alternatively, there could be a dedicated item that could get you out of the fight when consumed, while staying in the fight for more than one minute could start stinging everyone, just like the ring damage.
Looting stuff during a fight could happen in two ways, either having a ghost of you shown in the main dimension and the items that you get to disappear for everyone or having a duplicate of the crate in the parallel dimension, leaving the one in the main world unaffected. The deathbox of the defeated player and all the items dropped during the fight would be teleported back to the main world when that fight is over.
Abilities could be available only in fights, getting teleported with you, unless they are support (Lifeline's care package, Pathfinder's zipline, Octane's jump pad, etc), when they should either be available in both dimensions or only in the main one.
Throwing grenades, artillery or airstrike flares, triggering a gas can or electric fence should start the fight automatically with the player that's closest to center of that area. However, they should remain or have duplicates in the real world (unless created or thrown after the fight has started) slowing, blinding or shaking everyone else, but without doing damage unless it's a player involved in that fight. If any of these are still available or in effect after the fight ends, they should get teleported to the main dimension.
A form of 3rd partying could be kept, by having a portal opened a few seconds after the fight has started. Another player can then get in the fight, but with more unknown variables, while those already in the fight could get a warning message saying that another player has joined the fight.
This idea of locking the fight between 2 instances could be applied to team mode as well, although there's more tactical stuff needed for teamplay and it could be affected (fight stealing).
The above is inspired by Destiny 2 game.There, you can't join events that require more power than you have, making the enemies in that event immune to your bullets. You can also see regular enemies vanish when a public event starts happening in an area, while secret areas and portals take you to instances of missions that are available only to you or your team, any other player going in the same direction with you disappearing. Things happen really smooth in there, with no loading screens or anything else to break your flow.
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