Rather than a traditional "time limit", if long rests could only happen on a few set locations, and certain cutscenes and story developments were to happen after key quests, the "time limit feel" would still be present without having an actual time limit. This probably also means that character progression has to occur outside of camping too, but why the heck wouldn't it?
What is the logical sense of limiting rest to specific locations in a situation where you have fast travel available with points at every step.
You could also limit fast travel to also be in set locations to make this suggestion work, like how BG1 made it that you had to leave the Map at the edge to fast travel.
Personally I don't think Timelimits would work for BG3, for them to work for me it has to be a core part of the experience, like in say Deadrising, but with BG3 I feel like it would feel almost tacked on and stifling. But I suggest it as an options that it would work for cause a lot of people want it.
Only if you limit fast travel to the current fast travel points it still won't matter because they are close enough to each other that you will lose max 1 minute more.
This method of restriction will not change anything.
At most, the game will get a little more annoying the moment you have to step back after each underdark fight.
For this to make any sense, 2/3 of all runes would have to be removed, which unfortunately would aggravate another problem, namely backtracking on empty maps.
Of course you also have to redesign a large part of the fights
