I think the types of enemies will definitely need to scale with power level.
Things like vampire hunters are likely to be threats up front, and work up to mid game with various special abilities they acquire on cooldowns, but ultimately as players reach upper echelons of power stuff like other undead and sorcerers are likely to be the only sort of reasonable threat just speaking as traditional RPG progression where you acquire power over time.
This is especially true with games as a service where it's impossible to balance new and old players properly unless you go the TF2 route and remove all traditional concepts of progression.
I think however, that a faction of the humans probably should go full blown heretic and utilize the creation of undead from their own ranks to battle the undead in a nasty Faustian bargain, or perhaps that some of them learn to bind certain undead to their will and such (ie summoners and such).
Consider at first a summoner might not even been in the spawn pool, then when they do they start with a couple of shades, and eventually at later levels command a squad of full blown wraiths, etc.
We also presumably have external threats like demons, angels, werewolves, dragons and stuff to consider.
Plus we can't discount the fact that undead always have some various weakness humans can exploit, though relying on that over the course of 7k+ hours of play is likely to grow very tedious.
I don't know exactly what is planned but I think the way to pace it out is to make upper level abilities available to characters that transcend ages, ie, you can get X, Y and Z power set and expand and grow them, but only to A, B and C level, while more potent disciplines become available over ages. Given that the notion of branching narratives and social media integration/feedback is promised in some shape, the game should be replayable so vets can experience the new age with a new character along side their friends just joining up, and also swap to a character that is more potent from a prior age when they want to chase vet content.
Then once that is established over a couple of ages, newer players can have the option to enter at the stage they would like. This also lets apocolypse balance each stage of the game rather than the whole game at once. It's still a tremendous job, but having a finish line for maximum potential of a character per season is likely the best route and then keep people playing with events, PVP and timed exclusive cosmetics tied to various achievements for that age.
This is all just speculation though. I have no idea what the intent is.
Overall it really comes down to how expansive the narative is for the player, what the upper echelons of power look like early on in the game, and of course their personal vision.
One thing I really do want to see more than anything though, is enemies growing in power, getting more cooldown abilities and tactics to use as we grow in strength. While I can say warframe is a great game in many respects, it has taught me a lot about game design in so much as you can't just jack the numbers to make things more spongey and expect that to hold player focus and engagement because there is a finite end to the learning curve, yes jacking up the numbers needs to happen, but it can't be the only thing. The best stories have great antagonists that grow and change, and seeing enemies grow over time as we grow in power and become a bigger threat to the living I think is ultra important.
Enemies need to be more than melee attack, range attack and grenade on a loop, they need special talents that make them worthy foes of the undead, otherwise the game quickly turns to grind and gets monotonous.
That said I have a good degree of faith in Denis' vision. If we go all the way back to LoK we can see he's smart enough to implement drawbacks to player gains, a fantastic example being the different weapons Kain acquires in blood omen. For example, the flame sword was super powerful but also killed your blood sources, this same sort of trade off needs to happen with abilities. other examples exist too, where certain abilities are niche while others are general use, while some attacks are ineffective against certain enemies and potent against others.
Combine that with decent enemy AI with enough tactics to counter us, throw in a mix of diverse enemy types and you have something that is likely going to need to be more than just button mashing.
Additionally I'd like to see a mode adjust button for players where they can decrease or increase the difficulty from standard for more and less potent/quantity drops. This would affect them individually rather than the group, since each player will likely get their own loot pool from a chest or enemy drop. This would be more where the numbers cranking would be easy to affect, ie, players in easy mode get -50% damage taken and cost to abilities but see a 75% drop in item quantity/quality, and the flip side, an increase of 25% in quantity/quality for players that take 50% more damage and ability cost and similar.
In this way players can tailor the game to their personal needs rather than fighting the forever uphill and never ending struggle of casual VS hardcore and all while still allowing anyone to play together. This is easy enough to configure and you can just make a slider that the player can only scale in a hub (ie, not mid mission).