Just throwing this out there. Haven't spent more than 20 seconds thinking it through, so feel free to shoot major holes in it.
Movement: You can move up to your speed. If you pass through the threatened area of a melee combatant, your movement ends immediately. This requires a move action.
Cautious Movement: You can move up to your speed. Your movement does
not automatically end if you pass through a melee combatant's threatened area. This requires a
standard action.
So, there's still a cost/benefit analysis to be considered. There's no flat out "You can't do this." You can still move around the fighter if you want, but it may be less effective/harder than simply confronting him first. But there's no extra rolls or extra actions to be made. It's a little less granular/simulationist than OAs, but also more mechanical/less fiat than "You just can't."
And yes, it means if you don't
know there's an enemy in the way, you probably won't select to take a cautious move, and it'll be too late when you discover he's there--but that makes sense to me.
One could do something similar with ranged attacks and spells, in terms of bumping the attack/casting time. Precisely how would depend on whether there are "full-round" actions or not, but it could be done one way or another.
Edit to add: There could, of course, be additional rules/abilities where fighters and other defenders are involved. This would just be the basic foundation and the general rule used most of the time.