Really? Name me all these other Opportunity Actions that are NOT just a trigger for an MBA. ALL of the ones I can think of entitle you to an MBA.
1) Every Shaman ever.
2) Pre-errata Blurred Step. I both won and lost a bet over that guy. This one even answers the 'Opportunity Action that doesn't involve an attack' challenge! 2 for 1!
I didn't even need to crack open a book, right there's two, and they're both ubiquitous at-will class features to boot! Every member of those classes has an answer that satisfies your challenge.
One of those was certainly in development before the PHB came out.
Any other requests?
The EASIEST way to accomplish that would have been to declare an MBA made with an Opportunity Action to be the definition of "Opportunity Attack" from the start. This does not confuse anything or create any void in the rules whatsoever.
Except... you've just removed opportunity attacks themselves, i.e. what we define as an opportunity attack. By making 'Opportunity Attack' into 'An attack made with an opportunity action' you've then removed the 'Thing triggered by moving or using ranged/area stuff adjacent to your enemy.' So you HAVE TO FILL THAT VOID.
So you have this new thing, you call it, I dunno, Reactive Strike. Name doesn't matter. So now you have some things that generally affect opportunity actions, and specific things that affect this Reactive Strike thing. Now, you might want to have things that ONLY affect reactive strike, because you don't want them for general use by, say, shaman spirit boons, or whatever. You might claim this is inelegant, but that's simply not true--sometimes you want specific powers to get bonuses that general powers do not. The martial power source has an entire section of MP2 devoted to this concept.
So... you have some abilities that key off of reactive strike, and others off the general opportunity action. And... you might have some triggered attacks that you want to key off the first example that aren't reactive strike.... so the elegant way is to simply say 'You may reactive strike when (new trigger)'
So... to recap what happens after you change the system:
You have things that key off of opportunity actions.
You have things that key off of the specific opportunity action that triggers from moving/ranged/area in range
You have things that also benefit from the above but are not that specific trigger.
In other words, the exact same situation you have now. You have: Opportunity action-keyed rules, opportunity attack-keyed rules, and every trigger for an opportunity action in the game can -elegantly- decide which categories apply to it simply by adding or omitting 'as an opportunity attack' to their text. That's how it works NOW. Everything intended to work with the same stuff as opportunity attacks DO, simply by keying directly off opportunity attacks, rather than opportunity actions. (Heavy Blade Opportunity, for example)
This is important, because it's clearly intended (as an example) that Fighters don't get a bonus to ALL opportunity action-class attacks... just the opportunity attack itself. But... there are alternative ways to trigger, and effects of, opportunity attacks, and they're explicitly defined.
This has nothing whatsoever to do with it. The fact that the rogue for whatever reason suddenly can't do squat with his OA whereas he's supremely deadly with his dagger the entire rest of the time is a whole different thing.
As a nitpick, most melee rogues are Strength-secondary. OAs are hardly a problem for them.
More importantly, many fighters are better at planning their attacks than sneaking attacks in as a matter of reaction. Why must they automatically be equal at both simply because they happen to be good at one?
What it comes down to tho is this lauding of 'consistancy' in all things like it's a design goal... it's not. The design goal involves simple rules, with complexity coming from specific exceptions to those rules.
Exception-based design is diametrically opposed to the concept of universal consistancy. Consistancy and exception are antonyms.