Hipolipolopigus, a few toughts.
Skyrim (as well as Morowind, Oblivion, Fallout), keep track of things like dropped items, moved objects, cleared/accessed areas, quests lines, killed npc's, and so on.
Wouldnt it be a titanic task, to force any system to recognize each of those variables on each of –let's say– two players?
Why not an area, like a dungeon, that could not be accessed without co-op, like an instanced dungeon (ala MMO)?
With a system similar to Oblivions arena, where a predeterminated set of armor was forced on your character, you could skip armor types/states/quality.
basing the reward on a clearance/number of enemies score, and avoid the implications of introducing external issues to normal Skyrim...
I read what some people wrote to you regarding the car/boat anology, and I agree. But I believe most of it could be circumvented by a new idea of execution.
Cheers.
The idea is (Currently. Subject to change) to have a save that is invisible to Skyrim and that can only be loaded up with the Client (Whether via console or via some SKSE invocation). How the save is used depends on how the game is run. If it's a Client, then it's only purpose is to store Race/Gender and possibly location (Though we may decide to reset this upon entering a hosted game). If it's a Server, then all relevant data will be used and Synced to the Client worlds as necessary.
How would you ensure that all players had the same version of the game? (And it would need that.) And the mods? What about ensuring that one player isn't using the console to "cheat"? How will you handle rendering the remote players? How do you handle who the mob is currently targetting? How would you implement equitable loot sharing? How much extra lag, due to the above items, would you introduce? How do you intend to have the different players know WHO (the IP address) to connect to?
Of those, the rendering of the other player is the must have thing. Oh, and make sure it's all synched..including the exact same mob on EVERY machine that is connected. You would have to account for machine A spawning a Frost Dragon while B spawns a Blood Dragon. There are many more questions that would have to be answered, but they all fall into the same general category of how will you take a single player game and transform hard coded single player logic and rendering code into multi player?
An ambitious project, to be sure. Yet you haven't shown (though you are recruiting for others to help) that you can address these issues. If I could get you to answer just ONE question, though, it would be this: How do you intend to transform the single player game mechanics into mutliplayer? And you can give me the technical answer...I can take it. I think you should start with the idea that was presented by another poster....write a mod that opens a chat window so that you can chat with others. Not even a logging feature, just straight text chat IN GAME (ala the MMO style). Once you have that accomplished, where several people can connect and chat, you might have a chance. You'll need that functionality for the rest of the idea to work, anyway, yet it appears that the idea was dismissed out of hand.
I am skeptical about your project. I wish you luck, and you may succeed, but I have my serious doubts. To be honest, I don't think this project will even be completed, regardless of whether it works right or not. But that is my opinion. Take care.
Please don't expect us to have an answer for
everything. We don't know enough about the inner workings of Skyrim to be sure of our ideas, though we can easily speculate. Responding where I can (They may seem brief and somewhat cold, sorry!);
Game Version; Easy enough. Our SKSE plugin will have a set version of Skyrim that it will be compatible with. This data will be transmitted to the server when a connection is established and the connection rejected/accepted based on this.
Mods; Read sArchiveList and sArchiveList2 from Skyrim.ini. Sort by name, compress, send. If mismatch to Server, refuse connection.
Cheats; Not 100% sure, though it should be possible to maintain a list of "bad" commands on the Server, hook when a client enters a command and send the entered command to the server. Force kick if it's blacklisted.
Remote Character Representation; This is where my idea of a "Mannequin" or "Puppet" NPC comes into play. Each client will represent the other players with these. When a client successfully connects, their race/gender data is sent and then set on their Mannequin(Technically Mannequins, but let's keep it simple

). When position/orientation data is received for player [x], their "Mannequin" is set to that position/orientation (Not streamed, sent when it changes). Same for Animation data (As much as possible, not 100% sure on this...).
Mobs; Mobs should detect the Mannequin NPCs as hostile (Perhaps in the Player faction?) and attack them based on Skyrim's normal way of doing this (All handled on the server). Damage data is sent to clients as necessary calculating for active buffs/etc.
Equitable Sharing; Not at all sure at this stage. Could be hard, could be easy. We'll see.
Performance Impact; Don't see how you can expect us to have an answer to this. Don't worry, we've fully benchmarked all end-product functionality in an alternate universe
/Crude Sarcasm ... Sorry
Connection; I can't believe this actually requires an answer

Friendo gives you their IP address, simple. Or, alternatively, a no-ip address. Resolving these to real IPs isn't hard.
Sync; We intend for the server-client (It does need the client to run the world) to run as many processes as it can based on input from clients while the clients have their logic suspended. Could be resource-heavy, but we just don't know.
Mechanics; Put simply, we don't intend to put mechanics in as-is. Some will require some hefty core-editing to work as intended. Others are just hooks. Can't say for certain without first looking at the CK (Beta test, pretty please gamesas!

).
Happy now?

This is all based on speculation of the inner workings and is subject to change.