TLDR AT BOTTOM OF LONG POST.
BattleTech is deep in this controversy right now. It's very interesting to see how it's playing out. Hint, it's all bad news for fans of multiplayer. Without enough player demand (almost all BattleTech multiplayer achievements rest at 0.01% completion rate, which is astoundingly low, there are more people playing Panzer Corps 2011 release multiplayer then there are people playing BattleTech 2018 multiplayer) Paradox (not HBS calling the shots anymore, remember) cannot, will not, and is not shifting resources to help out the beleaguered multiplayer scene. However dedicated that community is, they are busy crushed by their status as being massively outnumbered. Multiplayer doesn't get patched despite some extremely glaring issues, the big pilot overhaul took no considerations for the state of MP, and the big expansion pack coming is all singleplayer focused.
But I don't say that in a deriding way towards BTech and Paradox, they're clearly making the right decision on what to focus on for the overwhelming majority of their players.
The real problem with multiplayer is not one of technical ability to create things such as a multiplayer campaign, but the division of time and resources that is required and may simply not be available at the cost it would take to make. In fact there is a very interesting attitude about BattleTech that has been going around. A lot of people are actually really disappointed with the tacked on MP systems entirely. Not because they are tacked on, but the players who bought BattleTech for the singleplayer campaign experience actually see MP as draining dev time and resources away from the mode they play (SP) all for something they don't ever want to touch (MP). And with a game with a total population somewhere in the several hundred thousand (source Steam Spy, and total population meaning not active players, but players who have activated the game on their steam account through purchase or key activation) your dedicated multiplayer BattleTech community probably peaked at several hundred people, and after just a few short months of attrition has now easily fallen below 100 total individual people worldwide. The 'big' tournament going on is having a problem where players literally cannot practice lances, mechs, and strategies because there's no one left to play with. Lobbies just sit empty and unfilled for an hour. Takes longer to find a match than to play it.
But that's BattleTech. Panzer Corps is a very similar (turn base strategy), yet different beast. As clearly evidenced by any measurable metric, players generally come to Panzer Corps for the singleplayer. Not just singleplayer, but the campaign over solo scenarios. But lately there has been a pretty sizeable surge in Panzer Corps multiplayer, I think a lot to do with goose2's big tournaments. So though we can learn a lot from seeing BattleTech, they aren't rules and lessons that can be applied to Panzer Corps 2 on a 1:1 ratio.
A long time ago when I was still just modding, I dabbled in multiplayer campaign design. Not the technical aspects of setting up network infrastructure mind you, but the content facets. And multiplayer campaigns (I was going for coop personally) have to also get special treatment. In the same way trying to shoe horn a singleplayer scenario into a multiplayer one isn't good enough (I'm pretty sure the PzC tournaments rely on special purpose built scenarios ) you equally cannot just shove a singleplayer campaign into a coop or a versus setting. It too would need purpose built content. Coop needs to have everyone feel like they are contributing, if you just split the CORE allotment in a scenario between 2 or more players, and they each take their allotment and go off in different directions to get different objectives... how is that a coop experience?
I hope Panzer Corps 2 MP is at least equal to Panzer Corps, and hopefully will get a little bit more too. If there is expanded multiplayer elements, they would have to start small and test the waters. Is the MP community big enough? Is the work do-able with the time and resources available, or does focus on MP distract team members away from their other work and delay SP updates and improvements and content? Personally, I would go for a versus campaign first. Good Coop too tricky, requires too many overhauls to existing mechanics and ways of doing things. A versus campaign would take advantage of existing systems better. Both players start a scenario, they play it out to victory. Then they take their surviving forces into a MP version of the deployment screen, refit their army, and deploy to next scenario.
Phew just sounding that out is fright with controversy and potential problems. I'm still not talking about networking and connection and UI infrastructure, still just gameplay. If Scenario A is a massive blow out and one person gets obliterated and the other is barely hurt... how badly will the rest of the campaign play out? Will it be lopsidded forever against the person who lost because their force was decimated? If it's not lopsided, and scenario B is unaffected by the events of scenario A except for player 1 'won' scenario A... is it actually a campaign at all? Because at that point it's just one separate scenario played after another with no real meaningful campaign connection between any of them, for fear of outrageous snowball problems completely ruining the rest of the campaign after a first disaster of a battle.
Perhaps both sides can only pick 5 of their best units to continue the campaign with. So basically you have a CORE of 5, and a huge army of AUX. Winner of map 1 might have a slight advantage if they won and their 5 best units to carry over are in good shape, but 5 units isn't that unbalancing as your entire force carrying over would be. Or perhaps winner of map 1 won because they were more aggressive and cared less about preserving their 5 best units, while the other player is playing the 'long' game. Really babying those 5 units to reap the rewards of having 5 super elite units as the MP campaign continues. But then you have a problem of pre-knowledge meaning so much. If you don't know how long the MP campaign is, you might preserve your 5 best units but never use them because the campaign just ended.
Anyways this post got huge because I just went on and on... so I'll wrap up and TLDR: Multiplayer campaigns would be immensely complex undertakings. Not just for network and game infrastructure, but their content has many aspects that must be carefully considered and built properly, or you will just get a hot mess no one wants to touch. I highly suspect MP campaign design will be a highly iterative process too. Many possible ways will have to be explored, ranging from MP campaigns will full, limited, or no carry over. Then the community will end up deciding what is the best surely by virtue of playing that certain design more than the others.
I'd be up for taking a crack at it if and when the time comes, but there's no work for any of us campaign designers if the foundation of the game isn't there to start with.