In MMORPGs, one of the decisions that gets made in their design is whether or not to give player characters personal space, or as I usually call it a “bounding box”.
EverQuest had a bounding box. Two people could not pass directly through one another. This caused issues when popular NPCs, like bankers, would get mobbed by players resulting in people who could not get close enough to interact. And sometimes this caused huge uproars when it came to doorways and other tight spaces. It was not uncommon to see an ogre in the Plane of Knowledge sitting in the bank doorway being an ass and demanding to be paid to move. It had other uses too, on my server there was at least one incident of using an orge to block a passage way into raid content, denying competition to a second group to a raced spawn.
Of course, going with no bounding box at all can cause just as much issue. In World of Warcraft players have no personal space. This is great when it comes to the auction house because there will commonly be fifty people trying to crowd around the one NPC. The drawback comes in PvP combat. A spell caster has to keep his target in his field of view to cast, so in order to interrupt casting all a melee player needs to do is run right up, step through the caster, then step back. In about a second the target goes from being in front, to 180 degrees behind, back to front again. Even with good reflexes and high speed mouse control, its very hard not to lose the spell cast, and for a melee player with a slow two handed weapon he won’t even miss a swing while he two-steps the caster’s blast into interruption.
What I would suggest is that instead of strictly making the bounding box a property of the character object, also make it modifiable by the surroundings. Make it so that in a defined area, around an NPC or doorways to buildings or narrow hallways in dungeons, the player becomes “intangible” and other players can pass right through them, and while not in those areas, players have a seeming mass and cannot be stepped through.
And with this, I kick off my section “The Game That Never Was“, which is going to be a collection of ideas that I have about what would make the perfect MMORPG.