I think the xp system should stay the same. If you want to get the strongest all around you must kill/loot/pillage everything all around but it will also makes you no different from the bad guys you are fighting against. You will be just another power hungry tyrant. Which is sure thing a possible playtrought in a good rpg game but it should have some drawbacks. Like you get base negative attitude from npcs, guards attack at sight, some dialogues and quest are not accessable, npcs refuse to talk with you etc. This could be achieved by a karma system. Basicly if you do something moraly questionable like murder innocents or do a quest the "easy" way you will get negative karma. If you do good thing you get positive karma. This system should be character specific not party so in multiplayer there will be another reason to do quest differently and to turn agaist each other.
I don't know if the game is too far into development to implement such a system but i played some rpg games where karma system did miracles to replayability.
