My proposal is that for any combat encounter, I'll enumerate the visible enemies, and specify what direction (one of the eight cardinal compass directions) they are relative to the party's position. Also, I'll specify a general distance category (really close, close, moderate, far, really far). If a character has a movement of 40+ they can reach enemies at moderate range with a standard move. If movement is 30+ you can reach someone close in a standard move. If movement is 20+ you can reach someone really close with a standard move. With a full round move action, you can extend your range one category further.
If you close to within striking distance of an enemy, you are considered to be engaged in melee with that enemy (and vice versa, unless Reach is involved...we'll get to that later). If engaged in melee, you will incur attacks of opportunity for spell casting, use of ranged weapons, and other activities as specified in normal OGL rules. I would like to eliminate the 5' step, so we'll need to discuss how to withdraw from a melee engagement if desired, whether such activity incurs an AOO, and whether it would be a standard or full round action. There needs to be some mechanism in place, otherwise casters will be in trouble.
I like the idea of 'ganging up' as proposed by Walking Dad. I'd like to use a cumulative +2 attack bonus to each addtional character who attacks the same foe. But, I'm not sure this should automatically constitute an opportunity for sneak attack damage, as it does seem a bit overpowered to me. I think I'd rather rule that rogues have to move a bit more precisely (akin to the older concept of 'backstabbing') to get a chance for sneak attack damage. I agree that this should call for an Acrobatics check to avoid AOO's.
There is a mechanism in place. The withdraw action. Allows you to retreat from combat without provoking an AoO.
That leaves Tailspinner, haven't heard from you in a while. How are things coming with your rogue?
LOL, upon rereading what I wrote, it does sound silly doesn't it? I suppose what I meant is something more like a partial withdrawal. That is, if we do away with the 5' step, will there be any means for, say, a caster to safely cast a spell (without provoking an AOO) once he has been engaged in melee? Or should we keep the 5' step (or some equivalent...'sidestep' maybe'...the literal 5' step makes no sense in a mapless combat after all)?
I like the idea of 'ganging up' as proposed by Walking Dad. I'd like to use a cumulative +2 attack bonus to each addtional character who attacks the same foe. But, I'm not sure this should automatically constitute an opportunity for sneak attack damage, as it does seem a bit overpowered to me. I think I'd rather rule that rogues have to move a bit more precisely (akin to the older concept of 'backstabbing') to get a chance for sneak attack damage. I agree that this should call for an Acrobatics check to avoid AOO's.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.