Okay so here are my broad suggestions, no specifics yet as I have been browsing all my 3.x books for suitable additions that could work within the NWN engine. Obviously, at this point, this would be for the next Hak update, and arguing the power of the new added feats is moot because they are extremely powerful, and the wait and see attitude is plain wrong when we look at a game construed almost entirely by numbers.
For Fighters:
The introduction of Combat Stances, UNIQUE to the class, not unlike Expertise or Power Attack. What these could be would vary, as there's only a few in 3.x supplements as far as I could tell.
At levels 11 and 16, fighters should get a FREE UNIQUE feat, set in stone as an integral part of the class. Not something like a flat addition to damage, overwhelming crit or the like, but something for utility. Daily powers, possibly.
For Rogues:
I'll admit I've given this more thought because it's easier to come up with than Fighters, especially with what we're provided in 3.x.
Splitting Rogue feat selection to level 6, and level 16. The level 6 ones would be the weaker, less substantial additions from the current list, and the level 16 ones would be the better ones. To both lists I would recommend adding new feats to fit their power level, and, just a brief example, what this would look like with what we currently have and what I think is balanced -
Level 6 selection,
Slippery Mind
Opportunist
Skill Mastery
Level 16 selection,
Defensive Roll
Crippling Strike
Improved Evasion
Thinking of perhaps a couple of more feats to add to each list should be a balancing act, and perhaps encourage pure Rogues. I would go a step further and add a level 11 list, as well as the ability to take a rogue bonus feat at every normal level past 15, but I doubt that'd be received well (for whatever reason, as most of the other classes currently around have more ridiculous bonuses than any of those feats can ever count for).
Feedback would be welcome to this specific post.