Yeah, as others have said Megamek is fully playable singleplayer with a detailed campaign system adapted from the tabletop campaign books, plus a lot of original stuff. It generates scenarios, OpFors, maps, contracts, a hiring pool, a unit market, pretty much everything. They've recently expanded the personnel model too, now you can send off your guys to basic training, officer school, and a variety of war colleges for XP. They can randomly marry and have children too if you're interested in playing a multigenerational campaign. The map doesn't change dynamically, but factions appear and disappear and planets change hands according to the timeline. I played from 3035 to 3052 and the Clan Invasion happened exactly on schedule with all the same results.
The battle AI is pretty good for an amateur project too. It can't handle double-blind, but they've added an option for single-blind gameplay against the AI, so its units are hidden but it knows where all yours are.
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Here's an example of a 3 month Objective Raid contract I did in my last campaign. With StratCon enabled, there's a decent variety to the scenarios. They are tailored to your force to a degree as well, for example I wouldn't have gotten the low atmosphere and space scenarios if I didn't have an Aerospace wing. Some scenarios are absolutely incompletable though, especially early on, and sometimes you absolutely have to just take the L rather than lose your whole force on a suicide mission.