I don't think running out of new units is a real issue considering how many units FE has, and when FE switch comes out we'll get switch units for sure. New resource types make sense for the new upgrades as well. To me, this goes in hand with the powercreep - if new content is to be increasingly challenging, then player units need a fair chance to accomplish the new content, hence more powerful new units and upgrades for old units. I'm assuming here that we'll get upgrades later for some of the old units that got passed over this time, like Roy, Jeorge etc. I've played enough gachas/CCGs that my viewpoint here may be a bit warped though, since I'm used to the powercreep cycle.
My main concern is where they go with new game content from here. No matter how many new maps you add, they'll inevitably be cleared once you've reached a certain point in your collection, and after you've played them once there's no need to play them again. So that leads to a point where you have checkmarks on all the maps, all quests completed, and stamina sitting at full, where there's nothing to do with the new characters you want to roll because you've already cleared everything. I think Tempest was an attempt to address this by adding a "grinding" mode, but I've always been mixed on grinding events and the rewards are capped anyway. Chain challenges and squad assaults are good, but once again they're one time only deals. You could have self-imposed challenge runs, but it's not as interesting as in a regular FE game because of the simplified mechanics.
So to me, the solution is new game modes and/or new events where you can use these shiny new units you've just rolled. A tighter cycle of events just to keep players busy would be nice, even if they're as simple as tempest/gauntlet. Preferably they would release BHBs more often than once a month; I understand there's a constraint in pumping out GHBs in that you're also releasing a new unit but you can pump out BHBs comparatively easily. They could even put out reruns more frequently so there's always a BHB rerun running at all times.
As for more persistent content, there's only so much you can do here due to the nature of FE, and Arena assault already fulfills everything you want if you just want to face other player units (of course, you could try live PVP, but it'll just be people dancing around forever like in Fates PVP). But to me, part of the fun of FE is dealing with clever enemy unit placement in combination with the map, and AA is mostly stat checks and counterplay. One thing I've thought about is a Tower of Valni-type dungeon, where you have semi-randomized unit placement, but with absurdly min-maxed enemies for maximum difficulty - like Tempest Trials but on steroids. Then, at certain levels in the dungeon you'd have a treasure chest that randomly draws from a pool of rewards - maybe badges or seals or any of the new resources they're introducing. And maybe you'd get an exclusive character for clearing the dungeon. Anyway, there are a lot of ways to mess with this and it's one way of providing "grindable" content that isn't forced or time limited, but still has incentives for repeating over and over again and maybe some exclusive rewards for X clears.
Another idea I had, and one that will never get implemented, is to have a sort of user-submitted repository for challenge maps. Maybe you could have a contest that accepts submissions from the community with certain themes or constraints for each contest, and just release the winners at regular intervals. Or you could have some sort of interface that browses user-submitted maps and lets players rate them - this would require some sort of moderation though so it's highly unlikely.
Anyway there are a lot of ideas IS can use for new content. I'm excited to see what they will do with Book 2 but I'm assuming it's just new story maps, so I'm interested in when they'll think about new modes/events and such with an eye on providing more persistent content for players.
