Is it possible that offline mode is added in the future? Online only games are not futureproof, servers might(and will) go down for various reasons, devs can have cease and desist letter from IP owners, etc. After that all the hard work would simply disappear. Having offline mode would at least make sure the game would stay playable in the future…and would not force people to be online all the time.
London 2038 only has MP in mind.
Getting MP-only content (Stonehenge and Abyss) into SP would be a project in itself and require more work than the current volunteers have time and/or stamina for. That’s not to say it won’t ever happen, but it’s not a goal for the foreseeable future. Emulating MP in an “offline mode” would require server software to be running locally among other things, and releasing that would put 2038 and any other potential projects at risk.
The hellgateaus forums have some SP mods if you want to check those out.