The issue: if you aren't going to scale challenges to character level, but instead let players choose from wildly-variable challenge levels in a sandbox environment, there's an inherent unfairness in randomly determining the monster after the party has already embarked for a location. As an advocate for improv GMing, I should probably comment on this.
The odd thing is: this issue has never come up, because I have never determined the level of challenge after the players made a decision.
Part of the reason is that, if I'm going to make a roll to randomly assign a monster to a location, I can do that at the moment the players ask about the location, so I can always drop hints about the threat level when the players are prepping for travel.
But the main reason is that, in the sketchbox style I advocate, the emphasis is on sketch. I start with sketchy information about the surroundings, maybe one or two phrases associated with each map locale, and add more as needed. I may not have precise details about a location, but I have a general idea what it's going to be, so I have basic information I can supply to PCs when they gather information, and I can flesh out the details with random rolls as they zoom in on those details.
For example, in the campaign I was running at the beginning of the year, the PCs were in a small village with basically two small dungeons I'd prepped before play and suggestions of two more. There were also some hints of other locations on a wider area map, like the Devil's Tower and the Ivory Tower. One player said, "Let's go to the Devil's Tower!" but I said as an aside that not only is that area not drawn up yet, it's farther away and really they weren't ready for it. I had a general idea of what was in each location, so in theory I could have filled in information during play, but the main thing I knew was that these were supposed to be fully-active wizard's towers, not exactly a place for 1sr level characters. The main reason I steered them away from those towers, sadly, was because I figured they'd need to set up base in Port Skar (a large town) first and I wanted to do some rudimentary mapping of that bustling port before cutting them loose there. I specifically designed the much smaller village of Suskar as a training ground.
In the immediate environs of Suskar, there was a abandoned tower called The Elder Tower on the other side of the river. Unlike the mere ruins of a tower outside the village, The Elder Tower was partially intact and in a somewhat wild area. The players showed an interest in that tower before I'd planned for them to visit, but I had some general info the villagers could relay when asked, so they learned:
- That The Church of Urizen waged a campaign to stamp out the druidic faith,
- That The Elder Tower was in fact built as an old kingdom outpost on the ruins of a Chaos Druid shrine,
- That some dark vengeance of the Chaos Druids rose to destroy the tower, but the shrine was never rebuilt.
As they went to explore the tower, the PCs also encountered a couple stirges, then they met an NPC adventurer at the tower who informed them that his comrade had just died at the hands (beaks?) of a huge swarm of stirges in the tower's upper floor. That was plenty of foreshadowing for the flock of 20 or so stirges on the topmost floor, and the players came up with a plan which worked, even though on paper, the battle sounded like a mismatch (three 1sr level PCs without hirelings vs. 20+ stirges?)
The Elder Tower was risky, and ultimately dealt out a TPK, but that was after two trips. It wasn't the fact that I hadn't placed a single monster before the first trip that did them in, but taking one risk too many (and not catching on to the easy way to deal with the skeletons...)