I have a list.
New (possibly romanceable) party members, fully voiced with custom quests.
New lands to explore, with multiple quests and an immersive story.
Fully open-world, even if only post-completion. Wouldn't being able to explore all of Faerûn be great?
Homes that can be bought, earned, or built, and which you can have a family in.
New armors, new weapons, new cosmetics.
Custom races, custom classes, as well as what was simply left out.
Custom party sizes (6+) with the option of setting different party members in different places to have them wait until they're needed.
Increased/removed level caps so you can keep on growing.
New cosmetics with ways to change cosmetics in-game.
Randomly generated dungeons, like every time you go inside it's a completely different place. Could be explained by a powerful wizard or deity being tied to it.
Quest boards that randomly generate new quests, so you always have something to do.
Expanded and enhanced crafting system.
And, of course, a mod manager to make it easy to install, uninstall, or modify specifications of various mods. I'm a bit of a mod enthusiast, the mod manager is typically a must have no matter how many mods you want to install.