I adored KotC 1, and I saw this brewing, so I figured I'd post some thoughts.
1. I see Bluesalamander's got it in his mind to support modules. One nice thought would be to save skill/feat snapshots of each level of a character's development, and if a character was level 8 and the module was designed to for levels 4-12, a player could have the option to import the level 4 version of their character to the module.
2. New classes. Ahh, here I've got a lot to say. DnD has some serious weaknesses. For the lower levels (anything under level 12), it is VERY hard to come up with classes which reasonably replace the 4 primary classes; fighter, wizard, cleric, thief. Let's look at some of the alternate classes;
Barbarian - This one is the most suitable for subbing in for one of the 4 primaries. Gives up wearing heavy armor and a LOT of feats in exchange for more hit points, better damage output, and (sometimes) additional movement. Mechanically though, this is a fighter with fewer options.
Sorcerer - Also reasonable to sub in, in exchange for a wizard, especially in a CRPG. Limited spell selection in exchange for more casts per day. Also gives up a number of feats compared to wizard counterpart. Mechanically, however, VERY similar to wizard.
Monk - Interesting flavor, okay fighter once he gets to a certain level, but damage cannot begin to match 2h weapon specialist. For quite a number of levels, HP and AC simply not suitable to replace fighter.
Druid - One of my favourite classes, but who do you replace? Would you give up fireball? Would you give up spontaneous healing? Fantastic class to add to a party, IF you have the fundamentals already covered, due to the fact that druids have so many interesting options.
Bard - Interesting class, does many things, but past 8th level, simply looses out on the power curve compared to every other class.
Ranger - Decent fighter, decent flavor, but not really suitable to replace a pure fighter.
Rogue - Sigh. One of the primary classes, yet very disappointing. Fulfills required social skills and trap/lock handling, but in combat, is VERY weak if not fighting monsters susceptible to sneak attack.
Multiclassing is even uglier. To match the power level of the fundamental classes, multiclassing is generally a bad idea until 8th or 9th level. Prestige classes are pretty messy too; if you're a caster (cleric, wizard, sorcerer), any prestige class which does not provide full spellcasting progression is usually a terrible idea, and any prestige class which provides full spellcasting progression is usually overpowered. Most of the multiclass builds have serious "dry spells" where they are underpowered, and many end up as "one trick pony" builds, where they have one horrifically overpowered talent.
So, how to overcome this? In a tactical combat focused CRPG, rogue is no longer one of the primaries. Fair enough. I'm not certain that additional classes truly need to be added. I think your best route would be to peruse prestige and secondary class features and re-do them as feats. Then add more feats to the base classes (aside from fighter). Clerics especially, and possibly arcane casters could use more feats to allow for more variation in builds. If you went this route, you could end up with a lot more interesting variation, and I think it would be slightly less complex than adding additional classes & prestige classes. Don't be afraid to change the character power level, as what you're building here is not DnD- it's a tactical fighting game which simply utilizes the same underlying system which DnD uses, and DnD's power levels weren't terribly well balanced to begin with. If the resulting characters end up more powerful, the monsters can be tuned up to match. A key feature in my mind would be providing more interesting character development choices, which would lead to better replayability.
3. New items? One of the things I like the least about DnD is the original design of magic items & wealth curve. Going strictly by the original book & items is terribly terribly boring. At a given character level, unless your GM has seen fit to grant you more wealth than the standard, your best choices in magical items are horrifyingly boring. A +2 or +4 bonus for your primary stat; a +1 or +2 weapon, A cloak of resistance, enchant your armor to +2, perhaps an amulet of natural armor and you're done. For many years, this was "how it was". Not only did your wealth limit your options, but because the standard bonuses came on specific slots, you were seriously limited on choices for magical items which were more flavorful and interesting. The Magic Item Compendium, one of the last books published for the 3.5 system changed that. There are many MANY interesting items & effects, many with reasonable prices, and there's a system for adding "standard" effects (e.g. stat bonus, resistance bonus, etc) to the interesting magical items. Please take a look at this for inspiration on how to expand the options on itemization!
I think going to 5 party members would be fun as it would allow for a bit more diversity while hopefully not slowing combats down too much.
Those are some initial thoughts. I'm really looking forward to seeing KotC 2, and what you do for it, as I adore KotC 1. If you're interested, I can look through some of the classes and prestige classes for some of the more intriguing options.
Cheers
Ikarius