My main point (which perhaps I did not make clearly enough) is that in an *ideal* sandbox, characters faced with the option of fighting ogres or hunting vampires would ask "what sounds more interesting?" rather than "what is an appropriate challenge for a party of our level?"
Wait, what? A person might think it is more
interesting to hop into a UFC ring, than to practice some mixed martial arts or boxing at a local gym. Unless they are insane though, they are going to choose an appropriate challenge.
... But if players have to ask - tacitly or expressly - "OK given our current level, can we handle this monster, or should we come back in a few weeks or months when we are more powerful" I'd call it metagaming. ...
There is nothing meta about a character assessing approximate strength of an enemy, it is something that we all are inherently capable of to varying degrees and it only makes sense that sentient creatures in-game would also be able to do so. As to the "come back in x amount of time when we are more powerful", that is not something that should be asked or answered. There might be a way for the PCs to kill that giant in non-traditional ways for example. Or, as mentioned they could choose to come back when they have worked themselves up to that metaphorical UFC level that I mentioned. Whether or not it is weeks or years in-game for them to be capable depends entirely on how the game is run.
I would argue that it IS metagaming when a party that almost died fighting kobolds and giant rats a few weeks ago says "Yeah, lets go kill us a troll!"
Again, that is not at all metagaming if approached from a character perspective. The characters see themselves improving, on whatever timescale is used, and if they can kill a giant rat without batting an eye it makes sense that they would be more confident to take on stronger foes.
I got a lot out of this thread, haven't seen so many open world type people in awhile. Sandbox style games are by far my favorite these days and I have found 5e really handles them fine. Bounded accuracy has helped me a lot. I don't often get to held up on numbers, as it has rarely ever taken away from the sandboxy feel, so I tend to use them to create the effect desired. The fact that a horde of goblins remains a threat even to higher level characters (albeit a somewhat annoying one) has been really nice. That little goblin lair you left a year ago shouldn't be much of a problem now... unless there are a lot more of them
Or more often, the PCs know that there are goblins in the area, but the number they encounter at any particular time is not definite.
I have played some really simulationist type games in the past, either by rolling or actually determining where darn near everything is, but I found all that really matters is the "feel". What matters is that the players feel like their characters can try to do anything it makes sense for them to try.