It all depends on the licensing deal they would get from AT&T. Microsoft would want the games on Game Pass day one, but AT&T would end up making less money because of it. So a compromise of them remaining multiplatform, but also on Game Pass day one, could potentially work out for both of them. 

Chances are all of those would release as multiplats. WB is selling the studios and possibly the non WB IP's, but they are keeping the WB IP's, and will license those IP's out instead. Therefore if MS acquires WB Interactive, they will most likely have to let those studios finish the games they are already developing, and let them release on the platforms they were originally intended to release on (unless MS pays extra to get them as exclusives). After the currently in development games though, we honestly don't know what could happen.

-MS could acquire those studios and put them to work on IP's they already own or new IP, saving money on having to pay AT&T/WB for IP licenses

-MS could acquire the studios and get the exclusive licenses for the IP's from AT&T/WB for a limited number of years and do those games as Xbox/PC exclusives (for instance EA got the exclusive Star Wars license for 10 years back in 2013, we could see MS pay for exclusive rights to DC, LOTR, Harry Potter, Lego, etc. for 10 years)

-MS could acquire the studios and get the exclusive licenses for the IP's as above, but release those games as fully multiplat, like they do on Minecraft.

-MS could acquire the studios and get non-exclusive licenses for the IP's, meaning that MS can make DC/LOTR/HP/etc. games, but AT&T/WB can choose to license those IP's out to other developers and publishers as well

MS hardly has a monopoly currently, I don't think there is a single country on the planet who try and block this acquisition on antitrust grounds. The studios they've acquired so far were either MS 2nd party already (Playground and Undead Labs), or independent studios that were struggling financially and were looking to be acquired (Obsidian, inXile, Double Fine, Ninja Theory). WB is no exception, AT&T wants to sell WB Interactive, MS wouldn't be seeking to acquire them aggressively or something. 

Besides, Sony is eyeing acquisitions right now as well, with rumors they are looking into acquiring Leyou, which would give them Digital Extremes (Warframe), Splash Damage (Brink, Gears Tactics), SNK (King of Fighters and Samurai Showdown), and possibly Certain Affinity (they'd have to choose to acquire the rest of Certain Affinity's shares, as Leyou only owns 20% of them currently).