With all due respect to Mike Mearls, he is wrong. The action economy in 5th Edition is beautifully designed, and I wouldn't change a thing about it.
MM talk about wording of the rules with bonus action.
I'm very curious to see how he would write down the "cunning action" rule without bonus action!
Maybe a rogue will to able to hide, disengage, make a off-hand attack, all in the same round!
Until we see the most complex case we cant tell much about its 'better and simpler solution'.
His comments, IMO, are along the lines of, "I have a better idea than bonus actions now, but I can't put them in the game until we get a new edition and that isn't going to happen until players demand it - which we're not even close to seeing, yet."
The impression I get is that you would just make Cunning Action an action you can take. "As a Cunning Action you can make a weapon atttack as well as do one of the following: hide, disengage, etc'"
Two weapon fighting would be its own action. "As an action, you can attack with two weapons."
So, the player just chooses an Action for his character during a given round, from a short list of actions. There's no wondering about how a bonus action might be tied to it. No trying to figure out if you can use one of your bonus actions. You just take the action and everything you can do with it is built in.
Another example: A bonus action spell rather than referring to other rules would just say. "When you cast this spell, you can also cast a cantrip or make an attack" No need for extra rules where the bonus action spells have their own subsystem.
Honestly, the more I think about it the more I like it.
AD
I feel like the thread is a bit misleading on the idea of '6th edition' - the current product release process points towards a long run for this edition. I would be surprised to see 6th edition before 2025 - and nothing in Mearl's words makes me think he believes otherwise. His comments, IMO, are along the lines of, "I have a better idea than bonus actions now, but I can't put them in the game until we get a new edition and that isn't going to happen until players demand it - which we're not even close to seeing, yet."