JamesonCourage
Adventurer
I prefer it as a feat. But I also like more feats than once every 2 or 3 levels, too. So, I'm one of those "complexity" guys. As always, play what you like
To me OAs are simple. "Don't take your eye off enemies in swords' reach of you or you will be smacked". Don't walk away from them. Don't try to use a distance weapon or attack. And don't do anything else stupid. AoOs are a little more complicated but only a little as they also include "Don't try stunts you don't have special training for" like grapple or trip without a feat, or just standing up. And in my experience it's this slight extra complication of the AoO that adds most of the problems.
Pathfinder Beginner Box has a replacement for AoOs and a glaring weakness. And older editions of D&D had AoOs. The Beginners Box replacement is a "You can't do that" - you can't shoot in melee. Rather than allowing you to at the penalty of an AoO/OA. And older editions had what was an AoO in all but name for trying to disengage. The glaring weakness in the Beginners Box is that monsters can brush past the fighter to splat the spellcaster.
They should be an optional module.
If I understand correctly, 5E core rules won't require battlegrid and miniatures. (Is this confirmed?)
I'd say Opportunity Attacks are a prime example for the "option in the tactical module that you can drop in your game if you like that sort of thing".
I prefer it as a feat. But I also like more feats than once every 2 or 3 levels, too. So, I'm one of those "complexity" guys. As always, play what you like