Even if they can't do mods I think doing some sort of limited time "server opens at this hour then closes 8 hours later" kinda thing to condense everyone like with GTA and then also add some kind of story element or challenge. Maybe the server is limited to a 1kx1k area and split into two groups and they have to compete or something, and shit like the nether is disabled at first so people can't get endgame gear in literally 2 hours.
Creating an interesting map like floating islands or dungeons doesn't have to be as elaborate as Enreco either, you don't need a bunch of custom shit, just some interesting places to explore and some plugins which make the game harder so the autists don't immediately trivialize any kind of challenge for everyone else. Maybe redstone is in a far away floating island or something so farms are harder to make, maybe mobs become more powerful each couple of days by giving them potion effects.
Edit: The "regular SMP" shtick was fine in the 1st and 2nd servers when most of the nip members were total noobs who just made mud huts and flower gardens and like 3 people had full diamond, but after the IDs, specially moona and kaela came in, and everyone started copypasting farms from youtube in and tryharding the game it became a lot more boring. That kind of thing is what kills most SMPs anyway, nobody knows how to build because it takes 5000 hours to be anywhere near competent, and any challenge or exploration is void because every single resource in existence has been farmed and made easily accesible. It just becomes creative mode with extra steps.
