So, I've been on a lot of games and it has been a long time since I managed to launch a game successfully. That said, I've done it more than a few times, and the lessons I've learned are the following:
NPCs don't need to be around. But some manner of reason for players to get together does.
PCs need to be on-grid for the grid to matter, and the grid needs to not be bigger than your people. If you have 6 people logging in regularly, you probably only need a couple of rooms. If you have hundreds of people logging in it might be worth it to build out a mini-city of options. Most places build too much, and then people stay spread out. They are not 'forced' to engage.
It must be EASY for people to meet, and EASY for people to get rewarded, but rewards should go for making RP.
Make any upkeep tied to skills, if you want to ignore upkeep costs. So, lifestyles in CP or SR.
Mini-Games and hacking and whatnot, can be fun while you aren't needing other players around but they also encourage people sitting isolated and not playing with other players when Players ARE around. So, I say, adding those things is just dangerous to activity.
The last game I tried to build but got stuck on, because I'm not a coder of any kind, I'm a project person who knows some outdated MU* code. Was this:
Use a system where logging scenes is your metric for encouraging roleplay.
Make logged scenes that want 'credit' to be posted on a web portal that everyone has access to.
Then have something like > Per line/character/etc that a person roleplays WITH others (whether they are emoting a scene for missions/questions/jobs or just going to a bar and creating a social experience) gets xp/karma/rewards. Ideally this is weighted. New players earn more than old players with a sliding scale. This rewards ACTIVITY, not popularity, but people who spawn 1 or 2 sentences, in a clique environment have no means of acquiring more than the person who isn't in a clique but does a lot on-grid to generate fun.
Make it a policy that PRIVATE scenes, whether that means SEX (should never be public) or PVP, if allowed, (you might want to keep your private dealings here private till you do the deed) don't earn rewards on their own. People can TS all they want, but you aren't getting rewarded for it outside of the scene (it should be its own reward).
From here, I wanted to make it so scenes that need monetary rewards got published to the web, and people who read them and graded them for rewards got their own reward as well. Basically, paying people to do queues, and their 'approved' stamp carries their name. So, if there's anything nefarious it's all transparent and can be backtracked.
Staff's only role then is code and policies. If they want to run something, they jump on their character bit and use the automated systems. It allows players to sandbox play all they want. A grid that's tight, but malleable, a world that is rewarding, in a system of your choice.
Where I stopped was, I could get logs to go to a website, and I could get character sheets up on the website with privacy rules. I just couldn't get the rewards in game to link to the loggers I was trying.
The less staff needs you have, and the more players can feel rewarded for just logging in and playing (even if tiny amounts of xp/karma/etc) you are done. The system takes care of itself. Now it's up to your playerbase. You've just removed all the other barriers and 'work' out of the system. Plots are auto-rewarded for xp/karma based upon activity (equal to coffee shop rp) but you can earn monetary awards. You'd have to create policies around what's a good money reward or not, but that was easy in the system I was using because it was already laid out. And I had more plans to create tiers of reviewers, people with experience and whatnot, but I didn't really need that to launch.
I think that system would really answer a lot of the problems MU*s have had over the past few decades, especially with the lower activity levels and playerbases. It is one of the reasons people have moved to system-lite games, but those also don't have as much system encouragement to create addiction in logging in.
My really long 2 cents.