@BobGoblin You have some good points here. Lack of activity is a killer for MUs. I don't care how many characters you have connected, if I can't actively find RP, you're MU isn't active. I also think you hit on the heart of the matter when you say, "I've grown bored with games where I lounge RP around waiting for a staffer to maybe run something"
I think this is largely because players do not have easy access to plot. In many cases, they may not even know what plots are happening on the game. You need to have an easy way for players to see what's going on and to get involved in it. I love Arx's +plot system as a basis for this. Arx has several other commands that give players easy ways to get involved with and interact with plot that are also worth looking at.
Conversely, if you depend on Storyteller driven plot only, there will come a day when things grind to a halt, and return to what you're discussing above. Relying on ST-driven plot is inherently unsustaiable. The plot needs to derive from the game itself and have conflict as part of the system. Your game needs a way for players to drive their own plot, so that it bubbles upwards from the bottom and is not always imposed topdown. For example, coded blood scarcity on a Vampire game can give players something to spin their wheels over while waiting for the next plot action.
I do not believe that a module-plot system will resolve the issues you're talking about, because in the end, the players are still waiting around for someone else to do a thing.
A few other things:
1 - MUs do not give people reasons to interact. When designing a game, I think you have to think hard about what you're doing to push players towards each other and how they will find each other. Downtime costs, unique roles, factions, territory disputes, etc. All play into this.
2 - Games are designed in such a way that discourages activity. OOC Rooms, while popular, incentivize sitting OOC, by making social interaction available while doing so. They also disincentivize going IC, because doing so means players miss out on essential community building and social interactions. Chatting OOC is very important to your game, but don't let it get in the way of RP. Allowing people to make more alts than they can keep moderately active fills +who and +where with characters sitting idle and obscures where RP is to be found. This creates a culture of sitting idle, even among players with only 1 or 2 alts. Allowing players to accrue too much XP creates a game where a character can feasibly do all the things themselves and doesn't need to go to other characters. Limiting XP encourages specialization and keeps dinosaurs from souring the game for new players.