These are mostly going to have to do with what I've noticed playing the main campaign in NWN2 and MOTB. I've never had time to figure out how to use the editor...I wish I did.
1) In the main campaign of XP2, death has to be more penalizing. The system in the OC and MOTB (dead characters come back to life as long as one person stays alive) was too forgiving. Raise Dead/Ressurection should have an actual purpose. By the time battles were so big that using such spells seemed worthwhile in the OC, you were given a rod of resurrection anyway.
Cleric NPCs also should not heal you and restore you 100% everytime you ask. Those services would require payment. If you want to heal to full, incorporate actual, useful Inns.
2) Warlocks need better ways to control their abilities, and better AI. Hideous Blow should be a MODE. For example, if you want to use it you go into a mode that gives you one attack per round (since you get one blast per round iirc) and each hit you do adds eldritch blow damage. It's really not that hard.
Secondly, The default warlock AI should NEVER be running in and meleeing, unless you're in hideous blow mode. There's no point. If the warlock is not casting invocations their "normal attack" should be an eldritch blast. Period. Regular melee with a warlock (unless its with hideous blow) is pointless.
3) Better new classes and prestige classes. I know they are in D&D lore, but adding a sorcerer version of a class (i.e. favored soul for priest) is really NOT that interesting. Prestige classes/newly implemented classes that are interesting are ones that give you unique, special abilities that allow the class to be played with a bit of a different mechanic. Wu Jen is a good example. Blood Magus is a good example. Shifter is a good example. Wizardslayer is a good example (although that particular class shouldn't be implemented until fighting casters is actually a challenge). In otherwords, you need to add more classes like warlock, and more prestige classes like pale master, duelist, dragon desciple and warpriest.
4) Enemy casters need to be buffed...especially wizards. Wizards were the easiest fights for me in the OC. Half of them never casted summons. Most did not begin the battles buffed. What made casters challenging in BG (apart from having them actually do smart things like dispel your buffs and target individual party members) was the contingency/chain contingency spell. The AI does need to be buffed in general, but wizards need the biggest one.
5) Gear was not balanced in the OC and MOTB, and neither were enemies. I was able to become immune to almost everything early on in MOTB, and hardly ever took a point of damage during the last fight in the OC. Items that do things like give you 15 to all resistances or +6 or more to saving throws need to be extremely rare. Same with weapons with enhancement bonuses greater than 6. Crafting makes them too easy to create.
Enemies also need to be balanced, and encounters need to be more realistic. In the OC, it seemed that the undead scaled with game progress, but had the same names. That makes sense for creatures like vampires who can take class levels, but all generic zombies should be generic zombies. Not everything you encounter at high level needs to be high level, and actual high level encounters need to be tougher. I never had to reload during boss battles or end-of-dungeon encounters. The enemies were only "tougher" because they had more hp and more melee damage.
I understand that your character should "feel" more powerful as the levels improve, but the game also needs to be realistic. A level 18 wizard should not just "happen" to find a staff of the Magi right as he turns level 18. These instances were too common in the OC. Items of such power should be earned through particularly challenging fights. Koroboros (who drops the staff in the OC) was too easy. In actual D&D, the more powerful you get the more creative tactics you are capable of. Winning a battle should not involve overpowering enemies with pure force all the time. Implementing spells like wall of ice/stone/mud would make battle tactics much more fun. Or dimension door. Also, why can I never get my Mordenkainen's Magnificent Mansion?? WHY?!?!?!
Also, I must not be well-versed in bioware forum lore...who is this "Merlinda" mentioned in the earlier post lol.