I don't know.
I mean... yes, franchises are interesting and IPs important.
Yet, recent history shown that you can create new IPs in a relatively short time: just three years ago Horizon: Zero Dawn wasn't even published and now its sequel is one of the most anticipated games on PS5.
I think that in this case buying studios and their talent-competence is more important and if you look at the gaming division of WB you it contains eleven studios, with some of them large enough to develop more games at once. Some of them are historic studios such as Traveller's Tales or Netherrealms and, given enough resources (Microsoft has them) they could become a powerhouse, especially if you have a service like Game Pass that you want to feed with enough original content.
Under this aspect it is a match made in heaven, so I would not be surprised if Microsoft would look for an agreement.
As per the iPS, just think at a Killer Instinct game developed by Netherrealm, at a possible Mortal Kombat VS Killer Instinct, at a Too Human game by Rocksteady (that setting is still in the hands of Microsoft and it is actually a very nice and original one...) or a new Kameo by Traveller's Tales...
And I could go on and on and on...