My next Roll20 session of Dungeon of the Mad Mage is on Monday, and I am preparing level 2 of the dungeon. The general difficulty of having a memorable session in a mega-dungeon is breaking up the monotony of "open door, kill monster, loot treasure". That is made difficult by the design philosophy of 5th edition Dungeons & Dragons, and the adventure module that very much follows this design: Lots of small encounters rather than a few large ones. In my opinion that was a bit of a design overreaction against 4th edition D&D, which made for great epic encounters. By trying to be "not 4E", 5E overdid encounter design towards an endless slog of boring fights.

As a DM preparing a level in a published dungeon, and not wanting to completely rewrite everything, I can do two things: I can make a few of the better encounters epic, by making them harder. And I can make some of the minor encounters less of a slog, by offering non-combat options more visibly. There are a bunch of "factions" on each level of the dungeon; the murderhobo way of dealing with these factions by simply killing them all is actually the least interesting option.

There are very many different styles of running Dungeons & Dragons as the Dungeon Master. And different DMs have different talents. While I am unable to do voice acting like Matt Mercer, and my narrative description of places and events is hobbled by me playing in French, which is only my third language, my strong point is staging epic fights. It takes a bit of preparation, you can't just start reading the Monster Manual when the group is already facing the monsters. But by using clever tactics for your monsters, using terrain, and preparing a surprise or two, any sufficiently hard fight can be made epic and memorable. You want your players to feel that they are in danger, and be proud of themselves for overcoming this challenge.

Preparation is also the key to the second part of the plan, pushing non-combat options for the factions in the dungeon. There are some elements you can use for that in the Dungeon of the Mad Mage book. Each faction is described in a paragraph at the start of each level chapter in the book. Usually the rooms that a faction occupies are all having the same number, and then are labeled with letters for each individual room. The necessary preparation is to read all the entries, e.g. 1a to 1f plus the faction description at the start of the chapter, to work out what should happen when the group approaches. Given that these factions are usually comprised of "monsters", e.g. goblins, wererats, or drow, you need to be prepared for two questions: Why do the monsters not attack the group on sight? And why should the group not attack the monsters on sight? For example in one spot the den of Xanathar's guild is guarded by two bugbears. The bugbears don't attack on sight, because they don't know whether the group belongs to the guild or not, so they first ask for a pass phrase. And as the players found that pass phrase in a previous encounter on a dead bugbear, they might well remember that, and give the right answer. And then they probably feel clever for having done so, which encourages them to not directly murder the bugbears.

The overall idea is to create a flow in the game that isn't always the same. There are easy fights, epic fights, puzzles, and role-playing encounters. Of course you need some cooperation of the players for that. In the last session they encountered a harpsichord made out of bones, and the idea was that they would play a tune on it to open a secret compartment with a treasure. That ended up with the paladin taking his maul to the harpsichord and destroying the treasure in the process.