@Stabbey This could be an interesting solution despite being a disguised time constraint. There is one major problem and one major benefit I see here.
Scenario: You cleared the gate and sneaked inside. You executed everyone inside. Outside you have 18 enemies awaiting you.
The major issue I see here is -> Sooner or later you might end up against the wall with no possible solution.
If you replace the 18 super simple to kill goblins with a boss where your wizard is actually required there is nothing you can do to fight the boss in question. Or rather your mage will be borderline useless during that fight. That will be super difficult all of sudden and might become frustrating. Unless you balance the game around cantrips and melee characters.
You can eventually leave and come back for him later. That would make sense, you're not in power to kill who you want when you want at your level. And that's the wall I'm speaking off. In D&D it would make sense story wise to leave this fight, do something else and you're still playing.
In a PC game you're loosing content. If you have to skip some fights or have serious issues during them because " you're can't rest yet" that would ( based on the current form of BG3) become an issue.
Yes, i have no solution for this. Yours is the closest one to become a good idea BUT we would need to find something for the " wall" i mentioned.
I think my quote above accurately captures your argument. If I missed something, I apologize.
Can you elaborate how one would be "losing content" in this case? With @Stabbey's fatigue system, you gain fatigue through walking. Thus, you simply leave (sneaking if necessary) and do something else before you rest. It causes some time lost, but that is not terribly
a) It will incentivize you to be a bit more cautious in your spell usage in the future, which I think agrees with 5e's focus on resource management
b) This provides a perfect opportunity to go to camp, where Larian has a bunch of cutscenes/camp dialogues they want you to see.
c) After you walk away, rest, and come back, that encounter will still be waiting for you! You haven't "lost content." It has just been delayed.
d) Presumably you would go find an easier encounter to do before resting. So it doesn't even delay content, it just rearranges it. You now do B (other easy encounter) before A (hard boss encounter), instead of A before B.