I have a question for them. Why design arenas at all? The original had a couple. The new one was 70% arenas. It sucked. 7/10 Too many arenas.
They should go back and use a modified Gzdoom engine honestly. Fuck these graphics whores and their 10 monsters on the screen at a time.
As enemy AI and mobility improves, level design has to adapt to reflect these changes in enemy capability and give the enemies the opportunity to shine. One important thing to keep in mind is that traditional FPS level design hinges on the fact that enemies are incredibly immobile compared to the player. Levels here are always designed to put the player into a disadvantageous position. If the enemies could abandon their positions to immediately claw your face within the span of two seconds, then there would be no need for complicated enemy placement or level geometry because of how the enemy AI functions (consider how in Dishonored if you trigger the alarm all guards in the vicinity abandon their posts to all rush you, and once you kill all of them a good half of the level is now completely devoid of enemies).
If the enemies are designed around being highly agile and mobile, placing them into small labyrinthine areas or monster closets gives them no room to move and show off their capabilities. So, a need arises to design levels which both the enemy and player can make effective use of, resulting in less levels biased against the player in terms of level geometry,
and more circular arenas will arise as a result, something executed very well in F.E.A.R. More open, clearly observable battle-grounds also work better with projectile-based enemies, giving you less opportunity to kite or back up to avoid an onslaught of projectiles, whereas having plenty of opportunity to break line of sight (and thus, smaller more compact levels) is a necessity for hitscan-based enemies. In nuDoom's case, the level geometry's role is to give both the enemy and the player enough avenue to move about, which isn't possible when the level design is asymmetric as it would be in traditional level design.
Also consider that exploration and combat in nuDoom are two separated modes of gameplay, rather than one cohesive whole. Glory Kills and Chainsaw ammo piñatas made short-term resource management and exploring for supplies largely useless, because those systems will already take care of your needs for you as long as you execute them every now and then. The only items of any worth in nuDoom's item economy are then chainsaw fuel, armor, mega items, power-ups, and upgrade points, items which you can't readily sprinkle around without heavily devaluing them the same way you can ammo and medkit pick-ups. It's also why nuDoom has these crates which refill all your ammo in the exploration segments, and why upgrade points were instated to give some purpose to exploration (we gotta have exploration because this is doom lol) since regular supplies won't do any more. Actual ammo pick-ups are mostly present in the arenas, where you're more likely to need them the most on top of having enough reason to keep moving about (it's not like you're going to be shooting much outside arenas).
With an arena-based structure for the combat, it begs the question whether there needs to be any exploration in the game at all, and whether the whole tacked on exploratory segments should be dropped in favor of just teleporting you to the next arena Nex Machina-style once you're done. This is where I'd wish nuDoom to have made up its mind from the start whether it is trying to be its own thing or trying to be Doom in places that don't really fit. An unholy separation of combat and exploration instead of the tight fusion of both the original Doom had just doesn't work.
There's nothing to suggest arena level design can't be done right or that it doesn't have its own strengths, we just haven't seen a shining example of "THIS is how you do arena level design right" yet, because it sure as hell isn't nuDoom (for reasons tl;dr). It doesn't help either that the FPS genre is incredibly stagnant, so the only lens through which people can judge the quality of FPS levels are through the standards of the shooters already considered good (old-school), especially when this new game is called Doom to begin with. So anything that tries to do something different can't help but be the victim of misplaced expectations and standards, rather than be judged by how well it works within its own context.