There’s not a lot special when comparing a modern engine to an older one with your request specifically.
200 pop cap / player was chosen because 8 & 200 = 1600 entities which can move around at once is roughly a reasonable upper bound on how many units a ‘standard’ computer can handle.
You may say, but Rome total war etc have more units!? and this is correct, but the units in aoe games are very individual, and require much larger overhead than squad or ‘army’ based games. Despite advances in the speed of processors etc, because of the way multiplayer still needs to be implemented, its difficult to get more than 1600 unique entities all pathfinding and moving around, checking ranges, auto-attacking etc. (you’ll note the original value in aok was 75 per player)
From what I’ve seen, I suspect the 200 limit is not hard-coded, but variable, and that you’ll be able to ‘break’ the game and have a 8 player match with 1600 pop each, but it would be laggy and unplayable for most people, unless all 8 players have top-end computers, and are likely LAN.
On the plus side, 1v1 with 800 pop each is likely to be perfectly possible.
This is all assuming that the devs allow pop change, which I suspect they will, as its not difficult to code per se. However only a Dev will be able to answer if they include that feature. As far as what to expect, I would look to the population system in age2 as an example of what age4 is likely to have.