One thing that I would like to see is more cohesion between the "hi-tech" levels and enemies and the "otherworld" levels and enemies...i.e. like in the base levels you would see things like shamblers and fiends in holding cells behind cage doors or forcefield barriers and grunts and enforcers standing guard...or perhaps base enemies leading shamblers by a chain and releasing them when they saw you like they were some sort of huge guard dog. If you look at the grunts posted at next to the perimeter of the slipgate entrance in E1M2, it almost looks like they are some sort of remote outpost of the base enemies of in Quake realm.
The base levels in Quake I were more impressive than the ones in Quake II. Even though they were "futuristic" they had a dismal feel to them with the dirty walls, pools of slime, rusted metal, and the dim lighting. Whatever intelligence the grunts and enforcers had seems to have regressed long ago, as grunts did not seem capable of speech and only growled as they shuffled along when they moved. While enforcers could yell simple phrases of surprise when they saw you, there did not seem to be any higher thought behind their actions. There seemed to be a sort of "wrongness" about the military base levels in Quake I that was not present in Quake II which was part of the gritty atmosphere of the first game. If I were to compare it to anything, it is almost like the "nightmare" world in the first four Silent Hill games (I refuse to acknowledge the rest of the games in the franchise since then, considering how much the Silent Hill series has declined as a result.)
Actually, I would not mind a further expansion of the Quake I base levels if they ever return to the first game for inspirations for future installments as long as they are still drab, worn down, dirt-encrusted, and inhabited by lobotomized soldiers like they were in the first Quake game in addition to the otherworld and its denizens.
Oh, and in future installments, the fiends and shamblers BETTER BE EYELESS!
P.S. what is your opinion on the nature of a shambler's exterior? Do you think they have fur like a bear or a yeti, or do they merely have palid, white skin? I always thought it was the former, just that the graphical limitations of the original Quake engine did not allow it to be shown but other people seem to think that they have leathery hides. I like the style of mouth on your shambler as seems very similar to what the original model of the game looked like only upgraded with current graphics as the shambler does not have much of a neck and too many other artists make their shamblers too "humanoid" looking instead of sticking with its rather stocky and bear-like frame.