Not sure if this has been covered before. I used the search and found nothing of it.
If you think multiplayer attempts would be a waste due to linking issues then read the bottom of this post.
In the third generation games, upstairs in every Pokecenter is the two rooms - the wireless and normal link room, the second being like in the traditional games.
In the normal link room, you move in real time with each other. If you open two emulators and put them side by side, you literally move at the exact same pace on each. It's very smooth.
If you use a walk-through-walls code and go into the Pokecenter when you aren't linked up, you can still access that room. If you try to use the trade/ battle machine, it'll disconnect you but otherwise you can freely roam the room and talk to the NPC in there.
What I'm getting at is it's possible to have two players interact on the same map without a problem.
So what would happen if say, you modified the FRLG game much in this way:
First, you block off the exits of Viridian City. Next, you go into the Viridian Pokecenter and into the link room to the right. Inside of this room, you remove the NPC and the trade/ battle machine and basically turn it into a normal map.
Some things might not function correctly such as pokemon appearing in grass, or going through doors and whatnot.. but from my observation of how the link room functions it's just another part of the game's map.
The room isn't different than any others apart from the fact that to get there in a legit-manner requires you linking up with someone.
But ideally, you could replace the first Pokecenter's trade/ battle link room with an entire copy of the game and be able to play through with another person. As for trading with them, every Pokecenter (that's within the link room Pokecenter) could be set up so there'd be no NPC blocking the door since you're already connected and you can just walk up and access the trade/ battle machine like regularly.
You'd just never disconnect after. If you wanted to disconnect you would go back to the place the multiplayer map starts (Perhaps through the south wall of the Virdian Center's second level/ Red's houses second level).
Now before you say "OMG NOOB THIS WOULD NEVER WORK OMG".
I just want to know why this wouldn't work exactly?
I've been considering trying to get something like this to work, but chances are it isn't possible or people would have done it by now.
So I want to hear the complications as to why it wouldn't ever work before I waste my time trying to do it. xD
I've read that a lot of people have trouble connecting emulators with Pokemon therefore never bother with multiplayer content.
However, it's extremely simple to set it up.
I've never gotten the wireless to work and can't link up correctly at all on the newer versions of VBA.
I've gotten into the room on the left in the Pokecenter but when attempting to make a trade - disconnected. I've almost gotten into the room on the right but disconnected beforehand. Same with Mystery Gift.
Then I downloaded VBAL 1.72. Keep in mind, I'm not entirely sure if the version I downloaded is the same as the one offered for download on the site.
I went to the room on the right on two emulators. Talked. Saved.
What do you know?
It linked up without the tiniest flaw. I was able to get in the room, make a trade and everything. It worked fine. It even worked for rom hacks very well.
I use it regularly with a friend when we play the games together.
It's just like connecting to someone's gameboy with a link cord. :]