An idea to throw out that’s inspired by another game in the interests of making the open world more lively.
At the moment when you take over a FOB or some other Base/Outpost, that’s essentially it. No need to worry about anything anymore and they’re just a vehicle/weapon spawn point, Marine-fillers for your Razorbacks, and fast travel destination (which, when you get the Wasp, may not even fulfill that anymore). When you’ve collected all the collectibles, killed the HVTs, and captured all the outposts, there’s really nothing to do in the open world anymore.
Other than a campaign story expansion that’ll inevitably lead to a larger open world with new areas, one thing I would like to see added is a system like Division 2 when it comes to the outposts. Essentially, in Division 2 you have control points like the current FOBs in Infinite, but they can be in constant contestation. Forces from enemy factions are always being sent out to attack/capture them or sending resupply caravans to reinforce theirs and your friendly forces do the same. Lacking our scripted UNSC vs Banished battles, I think this would be at least a decent substitute to provide that entertainment and to make the open world a bit more lively.
For an even simpler version, you can just have random Banished counter attacks to try and recapture the outposts. Given how the campaign will probably be expanding to the rest of Zeta Halo where we’ll hopefully be doing more battles with the Banished and reuniting with Lasky, Blue Team, or other notable UNSC forces while the threat of the Endless builds, it would be nice to see more of that growing conflicts with our FOBs being attacked by a Wraith tank supported by Banished infantry or a Chieftain leading a Chopper pack or something of the sort that you have to respond and fend off.
In the farther future, maybe have Flood/Endless outbreaks that take over and you have to put them down.