I realize this post is months old, but I thought up a compelling idea for a co-op system.
Better late than never! But yeah, in all seriousness I'm still wanting to thrash out this idea :)
Your proposal sounds somewhat more ambitious than what I was planning, it seems like you're thinking about how to adapt Essentials so that it any game could support co-op (with a little work on the events), whereas I'm simply thinking of creating a single game (more like a demo, really) to demonstrate the concepts. The main difference is that I'm assuming that we'll design the routes around co-op, cut features that don't work nicely, etc.
Here's some thoughts on your ideas, anything I don't reply to specifically I agree with :)
It involves the partner system, where player 1 leads and player 2 automatically follows, but both players can also split off and play on their own as well. This means it would require a splitscreen set up, or at least 2 separate windows of the game, one for each player.
Party Up
Players can party up with each other by having one player interact with the other. So for example, if player 1 talks to player 2, a menu will open up for player 1 with these options: LEADER, JOIN PARTY. They do what you'd expect. LEADER causes dialogue to show up for player 2 that asks them if they want player 1 to lead them. JOIN PARTY asks player 2 if they want to follow player 1.
While partied up, both players will fight in trainer battles, which will always be double battles. There could even be a check that sees if a trainer has 1 pokemon, and if so, to clone that pokemon to prevent the double battle from becoming a 2v1. Wild battles will also be forced into double battle scenarios.
Either player can choose to leave the party at any time by interacting with the other player. In the case of the leader, they would have to turn around and interact with the follower. Because the follower is almost always facing the leader, they can press the interact button to leave the party at any time. Some sort of dialogue box would come up and ask "Are you sure you want to leave the party? yes/no".
Multiple windows is what I had in mind for playing over the internet. I probably wouldn't bother implementing split-screen because having two windows is a decent workaround.
I'm not sold on partying up because I don't think it would be fun to give up control of your player. But I do
love the idea of double battles, so what if the second player can turn a single battle into a double battle by interacting with their partner while the partner is in the "!" animation/dialogue of the NPC trainer?
That said, it could be a useful tool for cut scenes because otherwise any movement-based events could end up looking weird; e.g. imagine one of the players is standing in front of Oak's lab when the other activates the "Wait!" cut scene, Oak is going to walk through that first player.
When players are not partied up, each of them can play the game through as you'd normally expect. However, there's a few checks that need to be in place to balance things out.
Scenario 1: Trainers. Player 1 already fought a trainer solo, but Player 2 did not. Trainers will need a check to see who defeated them, and if one of the two players have not, to challenge said player if they spot them.
Scenario 2: Same as Scenario 1, only this time, both players are partied up. In this case, if at least one player has not fought the trainer yet, the trainer will challenge both players.
I think I prefer NPC trainers only battling once, because that way if the players split up to do different things it's easier for whichever finishes first to catch up with the other (because they get to "skip" all the battles), and also this lets the party clear mandatory battles more quickly, e.g. on Nugget Bridge player 1 could battle the first, third, and fifth trainers, while player 2 battles the others.
Scenario 3: Item pick ups. When partied up, if the leader picks up an item, the follower will also get said item. When playing independently, each item will need a check, like trainers, that tracks which player picked the item up already. If player 1 picks up the item, it will disappear for them, but player 2 will still be able to pick up said item. Maybe there could also be an option for players to give each other any number of items when interacting while partied up.
Scenario 4: Same as Scenario 3, but only one of two players picked up an item, and the player who did not pick up the item is now leading. In this case, I think only the player that hasn't picked the item up yet should get it. Otherwise you could easily create a loophole that allows duplication of rare items.
I guess for consistency with my opinion on NPC trainers I'd make items be one-shot across both players too, but I'm not particularly invested in that implementation. I definitely like the idea of being able to trade items with your partner. I'd probably go so far as to allow player 2 to buy Pokéballs for player 1 if they've run out in a wild battle. I doubt it will come up in the typical play through, but I would feel pretty clever if I thought of that when playing!
Scenario 8: Saving. Both players should have access to saving, and if one player saves, it should save for the other player as well. Allowing players to save independently could work, but might be messy. If players are playing independently and one player is either in a dialogue or in battle, the save dialogue could tell the player attempting to save that the other player is busy, please wait.
I like the shared saves. If need be you could restrict saving to only when both players are in a Pokémon Center to ensure that no weirdness can occur.
Scenario 10: Trading. This would probably be easiest to do at Pokecenters, either by having both players party up and enter a trade room together, or just having each player enter the trade room at any Pokecenter and linking up that way. Battling should be able to happen anywhere though, just by talking to the second player should give you the option to battle them. That brings us to the last scenario...
Another option is to let the players share the Pokémon Storage PC. Did you have a reason for wanting to prevent trades at any time?
Scenario 11: PvP battling. Co op players will have the option to battle each other. These battles should not reward Pokemon exp to prevent grinding off each other. Selecting the "Run" option during battle will give players the option to forfeit. If either player's Pokemon party is fainted at the end of the battle, they will white out as expected. In the case of a draw scenario due to self-destruct, etc, both players will be partied up when they white out and are sent to the Pokecenter.
I guess with my implementation of items only being on the ground once this means that you could PvP your partner, white them out, and steal the item(s) they were about to pick up! I… am okay with that >:D
As for whiting out in general, I think I'd let the player wait for their partner to show up and heal them rather than go back to the Pokémon Center, if they want.
Note 1: Since battles are generally limited to 6 Pokemon per team, each player may need to either choose 3 of the 6 in their Pokemon party to use in battle, or have a hard limit of 3 Pokemon in each player's party max. Alternatively, automatically pulling the first 3 Pokemon from each player's party into the battle could work too.
Yeah, choosing or automatically using the first three sounds like a good idea. The other option is to give each NPC trainer a second team that they use for double battles which could have to up twelve Pokémon. The other benefit of different teams is that you could deliberately give them Pokémon/movesets that are good in doubles.
Note 3: While I initially thought it might be a good idea to allow both players to phase through each other when playing independently, I realized that it would look really awkward if for example Player 1 is idling in front of the Pokecenter nurse and Player 2 walks inside them to talk to the nurse. For that reason (and I'm sure there's other scenarios where this would look weird), players should be solid to each other when playing independently.
Agreed! Solid players could also have benefits for puzzles, and for making mandatory trainers.
