It doesn't really make sense in Blood's logic to have a door require a key after it didn't previously. Unless you're deliberately looking to make something strange, I'd avoid this. What will probably better suit your needs will be having the door 'lock' upon the player entering the room. There are quite a few ways in which to approach this problem depending on your style and choices. You could simply have the door lock. You could have bars raise/lower that block the door from the inside. You could simply make it that the door doesn't open from the inside without the use of a button. As always, Mapedit can produce almost anything you can imagine, but some things are far more difficult than others to pull off. I think you should first decide on what you think would be the nicest way in which to manage this and then we can take it from there. What mechanism would you really like that indicates that the door no longer works? What means would you like the player to have to use to exit after the boss is dead?
So there are quite a few guidelines for implementing multiplayer modes. These are the hard rules you need to follow for individual modes.
For co-op:
- Keys must be specifically marked for respawn type when 3: permanent. That reads strangely in English, but basically just look for the respawn section in the key's properties after hitting ALT + F6 and make it 3: Permanent. This means you will need to duplicate all keys and have them differentiate between single player keys and co-op keys, which is a skill it sounds like you've already figured out. This means that the key will remain as an item that can be collected repeatedly and won't disappear. It's extremely important to always be mindful of the fact that in co-op players will die. Always expect them to die and accommodate that. If the key acts as it does in single player and the player carrying it dies, the area it is meant to unlock will be inaccessible and the game will be over, which is no fun.
- Be mindful of traps, environmental destruction and so on that might prevent the player from traversing parts of the map a second time. Again, if a player dies, they should be able to return to where they were upon death. A bridge which blows up, for example, will prevent a player who dies from reaching the same point and their game will be over. You need to establish a separate ruleset in co-op which allows players to not get stuck. A perfect example of this is your boss carriage on your train. If both players enter and die, and the door locks forever, they will be unable to try again and the map will be unable to be completed.
- Not so much a hard rule, but to set up the player starting points in co-op you need to place eight (or seven, but I'd advise against being lazy for player 1) starting points. Give them Type 1: Player Start. Data1 from 0-7 will allow you specify them for each of the players up to a maximum of 8, of course. Mapedit is zero-based, so 0 is player 1 and so on. This isn't critical, but if you don't create them, co-op players will start on top of each other, which feels amateurish and lazy. Mapedit is kind enough to give the little flags numbers for an easy visual reference in 3D mode.
- Player start locations for BloodBath and Teams are determined with sprites of Type 2: Bloodbath start. Again you will need 0-7. These are important to distribute sensibly around your map.
- Not a hard rule, but a bit of good hygiene, in BloodBath and Teams players always have all keys. Key door markers should not be present in this case, since they do not offer information of any value to the player.
- Red and blue bases need to be set up for Teams. For blue team's flag, you will use Type 145: Blue Team Base. This needs to be assigned the reserved RX ID of 80 and its STATE must be 1: ON. Red team's flag will use Type 146: Red Team Base. This needs to be assigned the reserved RX ID of 81 and its STATE must be 1:ON.
Phantom Express uses Blood's iconic train horn sound at its start. This is a SFX Generator which plays on the map's start. The sound ID is 276.
Unfortunately, Blood's monsters can see through any sprite configuration you make, no matter how difficult it might be for humans or Caleb to see through them. This is a limitation yet again when making sprite ceilings, and you will need to be careful with your placement or think of a solution you're happy with.
Blood's monsters can't interpret sprite constructions as paths either and they'll spin in circles when they can't find a path that they think is suitable. They can of course step on them, but they don't know how to traverse them instinctually as a player will. Yet again, there is no real solution to having monsters cross a sprite construction without solutions that basically defeat the purpose of having that sprite construction. This is a limitation to be aware of your future map creation.
I am pleased to hear that you are planning to make a full episode. That is a very bold decision which will require a tremendous amount of patience. I am eager to see what you come up with and will of course be around for whatever issues may arise.