I recently played through a couple of Doom 2 megawads that strangely enough both shared the same theme, without me realizing it until after the fact.
First, based on
Saint_Proverbius mentioning it, I tried out "Perdition's Gate". It's an older megawad that prefers to use the smaller monsters and gradually and slowly introduce the bigger monsters, with Arch-Viles being rarely used. One notable thing it does differently is that it uses the end-of-level and start-of-level screens to try to tell a story on a level-by-level basis, but then I noticed that the story jumped one over, so you get the next mission's storybit for the current map. But ultimately it's too old and progresses too slowly, and the maps are generally simple and uninteresting (and WTF was going on with the secret levels?) until, strangely, the final maps pick up the pace. Strangely enough the Icon of Sin is on Map29 and not Map30. Map30 instead is an 'Escape-before-the-time-runs-out!'-affair, where you have only two minutes to reach the exit point before everything blows up. Except the Icon of Sin is still active and spewing out monsters in your path, and then there's the two Cyberdemons trying to prevent your escape...
Overall I don't recommend it. It has aged badly.
The other megawas is "Fragport", another story-heavy wad where Doomguy makes his way through area after area, before blasting off in a space shuttle to an artificial moon to blow it up. "Fragport" uses more of a "show, don't tell" manner of storytelling, with maps representing areas along the way, like the docks, then the ship you undocked in, then arriving at a beach (while watching the ship sink behind you), then heading towards a town, then into the sewers, etc. Sometimes it works, sometimes it's just silly. The secret levels are a bit of both - Map31 is a map experimenting with black walls, while Map32 is... something. Dull, mostly. Oh, and there are puzzles to solve. Light ones, but still puzzles, both in Map32 and elsewhere.
Unusually for a Doom 2 megawad, there is only one hell-based map in the entire wad - otherwise it's all man-made structures and areas. And the final few maps are spent on battle arenas, each one housing more deadly creatures than the previous. And, to my surprise, the Icon of Sin is also located on Map29 in this megawad... but to get to it first you have to traverse a labyrinth of ducts, in the dark. You'll have to use the map to get through it, but if you focus entirely upon it you miss an important clue given - namely that once you start Map30, you have ten minutes to beat the map until it blows up and kills you! No pressure then.
Except the Icon of Sin-fight here is one of the most intense fights I've ever had in Doom. Pre-present in the map are six Arch-Viles, but the Icon of Sin spews out so many enemies, in such a tight space, that it becomes a free-for-all fragfest in a matter of seconds. Good thing there are plenty of Megaspheres around. And the goal is to find and press all the switches in the area, to lower the barrier guarding the killswitch for the Icon of Sin. Press that, and the map ends.
And then Map30 starts, and you have to start by navigating your way BACKWARDS through that dark labyrinth of ducts, while on a timer... except something bugged out for me, and at the point where I'm supposed to be teleported out of the ducts to progress in the base itself, I get stuck in place, unable to move. Lovely. Nothing a little no-clip can't fix, but still. In the hallways that come afterwards, don't dally around. Find the right path, run like mad, kill everything that moves (and quickly) and don't try opening every door. Eventually you should reach an escape shuttle and end the campaign.
"Fragport" surprised and impressed me. It's showing its age, but it also isn't pulling its punches. That said, it is a bit easy as there's always plenty of ammo around. Worth a look if you're looking for something a little different in Doom 2, but it's not a keeper for me.