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Well, actually I think the "no locks & traps" concept works quite well for KotC. So I wouldn't want normal D&D rogue...

Here are my suggestions for a rogue character in KotC:

Special abilities:

"Spells": Controlling attribute is DEX. Either make this like the other spell systems or give them new and stronger "spells" automatically at level up.
There would be two categories/types of "spells": traps and alchemy (alchemy = bombs and enchanting weapons). All "spells" would cost gold when "casting".
Traps would always be only one field large (maybe larger at high level). I'm not sure if they should be visible or invisible. If an enemy walks on that field, the trap triggers. If an enemy rogue walks on that field, the trap is automatically disarmed. And they would be destroyed by damage spells (or maybe only specific spells like fire spells?).
Of course this would only be in battle. After the battle all traps would be disarmed.
If I may say so myself, this would imo be the only working (and fun) trap system in any RPG ever.
Bombs and enchanting weapons would work just like in other RPGs...

Use magic device: Just like in D&D, but only based on character level and intelligence (since there are no skills).

Insightful strike: Just like the Swashbuckler feat from D&D/NWN2: Instead of STR bonus, INT bonus is added to damage rolls, but only when using light weapons or a rapier.

If it's difficult to implement sneak attacks, they could get better critical hits instead...

If there were to be a Rogue (I think the game works fine without one), can you at least call him a Thief? Rogue is such a wussy, cop-out term for what the character really is: he steals stuff and he kills people by stabbing them in the back. I think management changed it back in the 80's because of the negative connotations of D&D, like how it causes teenagers to commit suicide, kill people, and sleep late on weekends.

The "thief" seems problematic if KotC 2 sustains its generally good-aligned, knight-based game-play. Robbery just doesn't seem to fit too well with that, and what's the point of a thief who never steals?

Why not take most of the OGL rogue concepts, adjust appropriately for the elimination of skills and the elimination of traps? I'd suggest incorporating the following:
Combat mechanics--retain as-is. Rogues receive evasion and sneak attack damage, for example.
Skill conversions--allow rogues to freely use spells from wands or scrolls (a la Use Magic Device). Maybe require a level check?
Allow rogues to avoid attacks of opportunity triggered by movement with a level check (Tumble).
Allow rogues to move through friendly and enemy spaces (but not end movement in them).

If you wanted to make them extremely valuable, you could replace trap detection/disarming with a danger sense. Give rogues a chance to avoid being surprised by an encounter which would otherwise surprise the party.

What I had in mind is the Assassin as presented in the Dungeon Master Guide (expanded to a 20-level class).

It is an arcane caster (with spells such as invisibility and true strike) with sneak attack damage and a few other abilities, like flanking immunity.

Allow rogues to avoid attacks of opportunity triggered by movement with a level check (Tumble).
Allow rogues to move through friendly and enemy spaces (but not end movement in them).

These sound good as special abilities.

I would like to see traps, not necessarily locks. Glyphs which set off spells, plus different types of damage, maybe even poison and disease would make the game more interesting. Plus the ability to disarm and find traps can be interesting as well. It would add to the game.

The other thing which a rogue could be useful for was for spotting secret doors or hidden items.

The problem with locks is it limits access to different places in the game. With a massively strong character, or a devious mage with the right spells I doubt doors, locks, or even chest lids would last very long.

How about giving rogue some chance to not be seen during combat (Hide, Move silently stuff) if they decided to go Stealth so they can sneak attack?

Also since he cannot use stuff like Bluff, Intimidate and other stuff maybe allowing parties with a Rogue get better prices in shops as the Rogue would probably have both Appraise and some social skill that in PnP can be used to lower/raise prices when buying/selling?

If you add it, I would advise to keep the game playable with or without that type of character - so it wont be a must in your party.

Well able to bash door and chest would solve stuff. Just taking trap damage is another

Sneaking in combat and sneak attacking is damage like any other. Better prices is nice, but can be lived without

I love playing hide and seek in my crpgs, so adding that would be cool. You don't necessarily need a rogue for that part of the game, a nice elf and dwarf combo should find most things. In the current game, you can have as much money as you want just by spending a few xp, so the bartering aspect of a rogue is not needed unless one of the game mechanics is changed. I'd say that if BS is going to add classes then he'll need to add characters to the party. Maybe move from 4 to 6. In general, the party was far outnumbered in KotC so adding a few characters to the party shouldn't be too bad, from a combat perspective. I'm not sure how much he'd have to tear out of his engine.

Spending a bit of XP is an understatement. That might lead you into fights where your caster is just missing the level that gets him his higher lvl spells and you cannot use those for that fight.