My suggestion:
- Outdoors only: You can go to the camp only if you are in an open area, away from enemies. Thus, it's not possible to skip to the camp from the Underdark, Goblin Fort, etc. In these cases, the party needs to go to a safe area for the long rest button to works.
- Sleeping bags: When they cannot go to the camp, the characters can sleep in sleeping bags. Just place the bag on the floor and click on it. It is also possible to sleep in the beds that you find in the buildings. Sleeping in a bag works just like a short rest. DOS has sleeping bags, so I believe it will be easy for Larian to implement. A message may also appear asking whether the group wants to light a fire or not.
- Time passing: Time passes when you sleep, and this can lead to complications for certain quests. I advise putting time into some quests, like saving Benryn.
- Random encounters: Random encounters are an important part of D&D games and this can appear in BG as well. When sleeping anywhere, you will have a percentage of being attacked by creatures. The type of creature depends on the environment where they are camping and whether or not they have lit a fire. Tests can be done to determine whether the characters were taken by surprise or not. To prevent the party from going to camp all the time, just put random encounters when they go camping as well. Perhaps to say that they were followed by some creature.
- Tiredness: After many hours without sleep, just fighting monsters, the characters can accumulate levels of exhaustion (PH, p. 291). This can be shown with them talking (“I'm exhausted, I need to rest.”), and at the interface, with a grayish color on the portraits.
- Nightmares: But how to prevent the characters from resting all the time? Simple, the tadpole does not allow. They will suffer from constant nightmares that prevent them from resting, and it's useless to sleep all the time. This can be shown with a narrator's dialog or the character saying that.
