I'm with
@GethN7 on the issues of the episodic structure of Doom 1 in terms of how it resets your inventory at the start of each episode. As he pointed out, it makes sense when going from E1 to E2, but by the time the third episode begins, wouldn't it make more sense for the Doomguy to bring everything he has to literally storm through Hell itself?
That being said, I do like the unique feel and style that each episode in Doom 1 had, going from the high-tech space station of Episode 1 to the warped ruined base in Episode 2 to the literal hellscape of Episode 3, and Doom II tried that up to a point with a different theme every ten levels or so, going from underground tunnels to city streets to hellscapes.
However, if I had the know-how, I'd do a total ground-up remake of Zombies TC for Doom II.
I know of the "Reanimated Edition" that's compatible with modern source ports, but I'd go for something that is more in-depth and covers the entire Romero trilogy, as opposed to the original TC being mostly just Dawn of the Dead with a few late levels based off of Day.
In fact, a complete reboot of Zombies TC is the kind of thing where the episodic structure of the first Doom would actually work better. Each episode would roughly correspond to a different movie within the trilogy.
Episode 1 would mostly be set in locations reminiscent of the original Night of the Living Dead, with woods, a rural cemetery, a farmhouse and adjacent farmland, maybe have a stage that's sort of like a small town in 1960's America.
Episode 2 would be focused on Dawn of the Dead with its suburban mall setting, obviously.
Episode 3 would be the dank claustrophobic army bunkers and laboratory from Day of the Dead.
If one were ambitious enough, you could even do a fourth episode loosely equivalent to Land of the Dead that's set in a wrecked cityscape.
I'd likely do overhauled enemies, both zombie and human, and a somewhat different weapons set from the original Zombies TC. I'd keep certain classics like the M16A1 from Dawn of the Dead, but I'd also add the Winchester rifle from Night of the Living Dead and have a Colt M1911 as the pistol since it's the sidearm used by both Bub and the soldiers in Day of the Dead.
If possible, I'd even include bosses at the end of every episode, each one a character from the movies
Episode 1 would have Sheriff McClelland
Episode 2 would have Blades
Episode 3 would have Captain Rhodes as the final boss
Hell, if it's doable, you could even have a different face for the main character for each episode if one were so inclined. The original Zombies TC had Peter from Dawn of the Dead replacing the Doomguy, so there is precedent.
The main protagonists would be Ben for Episode 1, Peter for Episode 2, and either Sarah or Bub for Episode 3.
TL;DR-They need to do a remake of Zombies TC and I need to get Doom II again. I already got the GZ Doom source port downloaded