I don't have a very solid handle on the mechanics you are envisaging this flexible edition having, but I tend to be sceptcial of the claim that a given ruleset can support both PF/3E-ish sandboxing, and 4e "legend-ising". Part of what makes 4e work in the way it does it that it has a multitude of big and small features that constrain the scope of the action resolution mechanics, and support strong scene-framing and scene engagement. These range from the resource recovery mechancis (surges, powers, rests etc), to the absence of "teleport out of the encounter" abilities, to the skill challenge mechanics, to the encounter building guidelines.Perhaps my miscommunication was to conflate simulationism and exploratory/sandboxy (for me, they go hand-in-hand). In my original post, I considered them compatible. So you can explore the game world through "pretend" simulationist mechanics, or you can explore the game world through the story, a flexibility which 4E doesn't and "Legends" wouldn't offer.
The beauty of that flexibility is that it can support both playstyles simultaneously.
Many if not most of these featuers, on the other hand, are exactly what those who prefer PF/3E-ish sandboxing appear to dislike about 4e.
Are you able to say a little more about how you envisage your flexible ruleset working? Even it's pared back all the way to defences, hit points and basic monster stats, for example, there will still be the isssue of resource recovery.
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