Wizards' Islands is the exception, the huge add-on with a large team that was actually finished. It took more than two years to get there, but at least...it arrived.
How huge? From the readme:
Intense and Epic Multiple-Paths Storyline between 20 and 30 hours.
Over 400 "Cell" Areas.
Over 3 Hours of music.
Tons of characters.
New creatures, items, spells (only for a boss creature atm.) weapons & armors.
And... more cool stuff.
Some of the "cool stuff" includes weather damage (so cold that if you sleep outside, you'll take damage at night), curses to some exteriors that prevent levitation, quie a few difficult quests, new monsters (giappele, blood wasps, dark knights, etc), and well-balanced unique items.