They’re very related. At the most basic level, it’s approaching the same fundamental issue from opposite angles. What I’m trying to do is to bring an offline fake server online, such that other players can connect to it and play with me. A “proper” solo mode is taking all the currently online functionality and moving it offline.
For example, they would have to build out a way to store, handle and process your inventory in your local client. Right now all your weapons, curios, etc, are all stored on Fatshark’s servers, and when you log in, your client pings the servers to get a list of what’s in your inventory, what items you have equipped, etc. Then, when you finish a mission, the server running the mission determines what your Emperor’s Gift loot reward is, and then it tells Fatshark’s inventory server “hey, character X on account Y just got item Z”, and when you load into the mourningstar, your client then pings Fatshark’s inventory servers and gets a new list with item Z in it.
For a full solo mode, they’d have to add functionality to each player’s client that does one of two things. It can either a) communicate with Fatshark’s inventory server to tell what’s in the user’s inventory and push updates to the server when new loot is dropped, or b) manage all the inventory on the client side, and the locally hosted server will update the client with any new loot or whatever, and then when you go to join a multiplayer game, the local client will inform fatshark’s servers “here’s the player’s inventory, don’t question anything in it”.
A) is relatively easy, as it’s “just” building out the inventory messaging service into the client, but it’s also not “real solo” because you’d have to be online for any progress to count, and that’s something most of the people agitating for solo mode won’t accept.
B) is a substantial undertaking, needing to rebuild the entire inventory system from scratch, and it opens an avenue for cheaters and hackers to come in and ruin online games with modded and glitched weapons, and that’s something all the people who play the game as it was “intended” won’t accept.
There’s also the C option, which is “modded realm”, and that’s basically B, except any character who engages in offline/solo/whatever you wanna call it play can never play normal online games or vice versa (and this is basically what Vermintide 2 has), but that still means doing all the work to build out the inventory systems in the local client.
And this is just for the inventory. There’s still character roster, cosmetics, havoc progress, penances, currencies, and probably a few more things I can’t think of offhand, all of which are around the same level of complexity and would require more-or-less the same sort of stuff being done.
Edit: To be clear, this is something that can absolutely be done, but it’s not nearly as simple or easy as people around here would like you to believe. I’d even like it to be done, tbh, cuz it would make my project way easier. I’d just really rather not have a 6-12 month content drought just so a handful of people can avoid being in the general vicinity of another human being.