The OA is mainly only ever going to be used by Melee classes. And out of the Melee classes mostly the Defender classes. The Defender classes all have some sort of ability that targets close enemies so it would not be a great stretch to just include the mechanics in those class' relevant powers.
The OA penalty on ranged attacks isn't really based on any real logic other than they are balancing the ability to attack at range. Why would firing a bow cause an OA where as not doing anything doesn't? Wouldn't it be better to strip out the ranged attacks causing OAs and just give the ranged attacks a penalty to attack adjacent targets, which is more logical anyway.
Summary:
Delete OAs.
Integrate OA mechanics fully into Defender Auras, etc. (or any non defender power that seems reasonable)
Ranged attacks gets penalty to hit adjacent (or other mechanic that seems reasonable.)
Benefit:
Give the OA ability to those who really care and need it. Pump it up if need be.
Take it away from anyone who doesn't care and can live without having to know about it. (Like a skinny bloke with a stick.)
Just a thought.
I would argue that the simpler rules are the consistent rules where ALL characters can do OAs when specific conditions exist, the current rule, vs making everyone learn that only certain characters can do it. Everyone still has to know the rule, at least SOME monsters will have to be able to do them, so the DM would have to remember which ones those are, etc. If, as you say, your rule isn't going to take it away from anyone that will actually need to use it meaningfully then IMHO it isn't any simpler.
If you want to streamline action economy in combat IMHO the way to do it would be along these lines:
1) Ditch the minor action - trivial things like drawing a weapon etc would simply become free actions. This gets rid of a whole slew of very unclear rules about switching and drawing weapons. It also removes the temptation to create minor action attacks, which are a whole other problematic aspect of the game. Sustaining of powers can simply be automatic.
2) Immediate Reaction - simply get rid of it. Anything that is now handled by immediate reaction can be handled as an interrupt. This will necessitate some adjustments to other mechanics, but nothing that the game can do now will be impossible to implement as an interrupt. This eliminates an entire class of action, simplifying the rules.
3) Opportunity Action - Just get rid of this as distinct from immediate interrupt. You can take an OA but you only get one per round. This also eliminates another type of action. It will limit the effectiveness of OAs somewhat, but not a whole lot and is no more or less 'realistic' than a character being able to make limitless OAs in a round. It tends to open up tactics a bit as well. Defenders can still have special mechanics to allow more effective stickiness if desired, which is no more complex than what we have now.
4) Ready Action - simply combine this with delay. There's no overwhelmingly compelling reason for needing both. Anything you can do with readied actions you can basically pull off with a properly designed delay.
This will eliminate a bunch of rules without materially changing the tactical flexibility of the existing 4e combat system. It would necessitate a lot of minor rewriting of powers and whatnot, but presumably '5e' would necessitate some changes to all of that anyway, so it is really a wash.
There are a lot more things that can be done outside of action economy as well, but all of that will need to be considered in the greater context of streamlining and cutting back on the proliferation of powers. Those are the other side of the coin. I'd considerably cut back on the numbers etc, but that's been discussed pretty extensively in other threads. I'd go with a pretty significant cutback on the types of durations as well. Most daily powers should either do something instantaneous or something with encounter duration. I'd reduce all the -2's and +2's in effects to some kind of 'chit' that you either have or don't have and shed at the end of your turn or else keep. Actually one way to handle that would be to just give out a specified number of chits. On your turn if you have chits you lose one. If you have blacks you get a -2, white chits you get a +2, etc. If someone gives you a white chit it just cancels a black or vice versa, otherwise it sticks around. All the other various effects can generally be rewritten as "the target does X right now" for the most part (IE jump, the target gets to jump right now). This eliminates a LOT of tracking nonsense. I suspect a number of conditions can be eliminated that way. If you have black chits you're 'granting CA'. Dazed is effectively the same thing. Stunned can still exist, maybe it is basically what Dazed is now. Combine Restrained and Immobilized. There are probably some others too. Slowed can probably just be jettisoned, it adds little to the game on its own. Weakened seems useful.
I think that and a general philosophy of just making situations where tracking is required and sticking to a very short list of non-condition effects would provide a considerable amount of streamlining. With reduced numbers of in-combat power effect options and probably a bit smaller numbers of overall power slots I'd make things like surprise and advantages from terrain more significant. Instead of devising all sorts of clever tricks based on action economy and such the players can spend some extra thought on getting the jump on their enemies and taking tactical advantage of terrain. That helps keep different classes well balanced, keeps tactics important, and gives the DM some more useful flexibility in encounter design at no extra burden during play.
I'm relatively confident that combat can be made 30-50% simpler along these lines and cut at least a quarter and maybe a half of the time spent on most encounters without any drastic reduction in overall quality of combat.
