I'm going to go out on a limb here and talk about something other people know way better than me. You know, for once.
Specifically the important programming based stuff I think the game needs to have.
Format
As far as languages go, I recommend java - we want something that can be run in a web browser, or offline as an executable. The idea for this is that games will take a while, people will want to access their sessions from more than one place, so ideally that should happen. In addition, "here's the game, play it" makes for a hell of a demo. This is obviously copying the Minecraft model, but that's because that is the best model. If it's online only, you can't play offline, if it's offline only, you have to play on one machine. It's also really cool to just link someone it and have them be able to play - the less barriers to entry the better, as a general rule.
Server
At a basic level, I think a client and a server will have to be created - the server contains a multiplayer session, the client connects to it. This wouldn't be a chain, it would just be a server that records the game state, the player data and any seeds for procedural stuff. For a single player session obviously no server would be required, as the data can just be written to a save file. That can just be a text file, I don't think it really matters if someone messes with it or anything - anyone wanting to play it legitimately won't do that, so encrypting is probably pointless. Assuming players had their accounts on a server, and had say a couple of megabytes storage space each, game data could be stored under their profiles, allowing them to access it anywhere. I don't know how big save data would be so I say megabytes, but realistically it'd be like 10 slots online, as many as they wanted offline.
Client
The client will have both single player and multiplayer functionality as outlined above, probably through direct connection to the server, which could be extended to an invite. Unless there's a system with public games where it's just an ongoing thing and anyone can play as any character (which would be fun) then a server browser wouldn't be necessary. From there (and here's the useful thing with using a modular language like Java) it's a matter of assembling characters, worlds and items out of the disparate parts that are programmed in, and then arranged based on which variables are set to what. I know that basically describes everything, but I just wanted to get the idea across that modularity is a core principle when it comes to writing this thing. So that means that when writing it you'd have basic walking around, hitting stuff etc then each new module would be a discrete part of gameplay - so the title module adds abilities, and the echeladder module adds stats. It means that the game is playable at every stage of production, and gets deeper as we go. So once the basic systems are in place, you can play a generic character walking around and stuff - about the level of interactivity of alterniabound, at this point it would be possible using basic mechanics to go to the medium, run around there, get to the battlefield using level warps and stuff. At that point perhaps a basic item system could be added, then some form of stats and so on. Actually once items and basic movement is added then we'd have enough to recreate act 1 more or less, so that could even be a demo.
More ideas as they come, I don't know how useful this, it's not incredibly technical, I just wanted to outline what we're aiming for in terms of the client/server/account architecture, and how to get there - starting from the barest minimum playable state and just adding on. There's really not a lot of interactivity between systems in Sburb, aside from like god tiers and sylladex weaponizing, so the implementation would be fairly clean, and could be iterated on as we go. I know it sounds like I'm just trying to rip off Minecraft if I say it like this, - starting basic and adding features until it becomes 'releasable' but it works because it's a good idea. And it's fun for those following development.