Note: Sorry for taking so long to respond, the forum crashed when I posted my message at around 1:27 am. If you have ever had anything you've typed disappear at 1:27 am you would be very pissed, I know I was; I'll try and make this of the same quality as it was before.
I'm not going to sugar coat this for you, I will simply lay out the Cold Hard Facts.
*pay no attention to the Coors light part*
What you are describing is Peer to Peer Hosting, basically rather than having a single host for all of your clients, each individual client will be able to host other clients. This also eliminates the need for you to host a dedicated server for the game because players who host the Co-op the game will become the server. All you need to do is create your game and include the necessary scripts and software to host the game. However, your are going to have a plethora of problems to address:
The necessary scripts will probably require special firewall permissions to allow the clients computer to host other clients. That being said, it will require every one who wants to be able to host a Co-op to set these settings
Furthermore, the necessary software for hosting will need to be installed on every client
While, you do not need a dedicated server for hosting, you will need something for matchmaking to allow clients to connect to other clients, most likely a dedicated server
Lastly, there are no known methods to achieve this in RMXP or RMVX
Do not fret though. I have a theory that might work if your able to deal with problems one and two.
Follow in order:
Complete your game (It will make things easier in the long run)
Find and Install an MMORPG script for RMVX or use RMXOS in combination with RGSS2 Conversion scripts found in Gubid's Tactical Battle System
Create an installer that: Installs your game and neccesary software, changes firewall settings for the game/program, and allows the user to select a host name(this will allow other clients to connect)
Create a website/chat that allows users to connect with each other so that they can host games
:o Good Luck
btw If I remember what else I posted I'll put it