Impactful PCs
Forget the hall of fame, but let the players exist in the narrative even post death. Publish in game documents that reference them, include referenced to them on signs or statues. If they build a castle and then eventually their faction bites the dust, let their castle remain as a haunted dungeon, with traces of their names and deeds. So on. It shouldn't be gaudy or cheap, but those that are memorable should be kept as important to the history.
Persistent Crafting / Minions / Projects
(Assuming eventually we can animate dead players) Say you vanquish your enemy, and animate him. This triumph shouldn't end just because the server resets or you log out. Make a way to 'store' these summons. The same could apply to hirelings. Or if you're a druid and your long term project to conduct a ritual to create a treeant sidekick finally happens, this creature could simple be 'stored' and retrieved until the sad day that it died. In a similar fashion, don't cap summons at a low number. A powerful wizard with 8 monsters summoned is cool.
Hirelings
Create a pool of low power NPCs that can be paid a big fat lump sum and kept semi-permanently on the payroll by the PC until the time limit for the gold paid is up. They should fulfill really trivial purposes like torchbearing or pack hauling, bodyguarding or trap finding. Perhaps make the pool finite. Throw in twenty fleshed out hirelings all over and let the players hire them when available. Remove them from plots, add more, etc.
Custom Spells
We've all got ideas. Add new spells via haks. A lot. This to me, is the most important part of any module on NWN. The game is 15 years old, there's thousands of D&D spells, add as many as you can. For all spellcasters.
Downtime Activities
Create spawn points that the players can select by traveling to that place in game. By choosing that location/residence he is given a selection of up to 3 downtime acitivites that provide certain benefits. So, say you go to the ranger cabin, talk to the NPC to set it as your save point. He says "OK, what downtime activities will you pursue while here?" And you're given three options. 1. Hunting. 2. Studying in solitude. 3 gathering herbs. Hunting would provide you one item from a table (that table is deer antlers, rabbit fur, wolf fangs and then a chance for rarities like a griffons talons). Studying would Grant you +1 Int, +1 Wis on login until reset (basically permanent until you choose a new location or activity). 3 gathering herbs, same as hunting, you get a random herbal crafting component.. fungus, plants, berries. Certain downtime activities can be restricted to certain classes and races or even levels, to limit the accessible power. Maybe the thief hideout let's you choose "murder for hire" as a downtime activity, which requires a level in assassin. This could give you a random sum of gold like 500-1000 per reset or a rare poison. I like the idea of player characters having life beyond what happens in game.
Wondrous Item Crafting
Whether by resource collection, numerical skill development, feat selection or otherwise, players should be able to craft powerful magic items. Circlets of intelligence. Staves, Rods, Magical Armor, Druidic staves and totems. Don't limit crafting to the mundane. And ideally, combine mundane and magical crafting so that one requires the other. Balance is the hard part, figuring out how to maintain powerful loot while not overpowering crafting, as well as how to not ruin crafting by only making loot valuable.
Spell Combos / Circle Magic / Ritual Magic
In addition to new spells, a system in which two players can cast spells at the same time to create a completely new effect. They may require some kind of focus, whether an item or another player. No idea how you'd script it, but here's the idea: Wizard A casts gust of wind on taget. At the right timing window, wizard b casts lightning on the point. The two wizards are flagged as "Combined" before all of this happens, the result is a thunderstorm. Knockdown on area as well as the druid spell lightning cloud.. then again this is probably unscriptable as a combo.