Well, I’m sort of a lawyer – I have a JD and passed the bar exam, but never actually got admitted to practice anywhere since my career has been in policy and advocacy, not suing people, and the closest I’ve ever gotten to IP law is some classes on the First Amendment (which is not very close as nearly all IP law is based on statutes and common law). So you are getting precisely what you pay for with this bit of speculation on my part!
First off, copyright doesn’t depend on publication – copyright attaches at the moment of creation. So this proposal, tentative though it is, would almost certainly be found to be part of the Infocom IP that Activision/Blizzard now owns.
As to how far that copyright goes, the black-letter law rule is that copyright doesn’t protect ideas, just the (artistic) expression of those ideas. That’s easy enough to say, of course, but where the line actually gets drawn can get tricky in practice. For the Frat House example linked above, there’s enough detail there that if you implemented a game that incorporated everything actually as proposed, that would clearly run afoul of the copyright.
On the other end of the scale, if you just thought “a frat rush week – cool, that’d be a neat idea for a game!” and made up your own scenario, it likely wouldn’t be found to be infringing the copyright just because in fleshing out the idea, you hit upon some of the same specifics. Like, you can’t do that scenario without including a series of pranks, and probably a scene where the head of the frat kicks things off, so those would in all probability be OK.
The middle case where you’ve got the same concept, and some things are the same (say, the car-in-room prank) but others are different (no “barf badge”, ugh) is where things get really fact-specific and additional legal doctrines will come in to help guide an outcome – and as mentioned, I’m nowhere near an expert so I only have the dimmest sense of what those would be!
Anyway bottom line is yes, Activision would likely have a cause of action against anyone who made that game (not Microsoft yet, since the merger hasn’t gone through), and they’d almost certainly win. The question of whether it’d be worth their while or if they’d do that is a different question, of course – there’s been a lot of IF riffing on Infocom’s IP over the years, including ones that incorporate clear-cut copyrighted material like grues, and to my knowledge there’s never been a peep out of them, so picking an implementation of a three-page proposal for an unreleased game to change that policy seems deeply unlikely (if MS does have ideas to exploit the IP post-merger, I suppose that might change, but it still seems like a very low change). And since the monetary damages from distributing a free game based on an unreleased idea would presumably be negligible, I’d think that worst case scenario all you’d be looking at is a cease-and-desist demand. So I’d personally be comfortable running the risk, but it’s probably a risk nonetheless.