I honestly don't want to add too much difficulty to it, but I don't want it to be as easy as it is right now. IMO the problem with resting right now is the fast travel system. If you had to get to a fast travel point in order to fast travel anywhere, including to get to your camp before you could take a nights rest and if there was a chance your nights rest could be interupted by some kind of random encounter attack on your camp you would have a fine system worthy of the original games.
Needing to walk to a fast travel point would start to feel like pointless busywork unless there was the chance for a random encounter or something. And in a system without a human DM to manage, random encounters have issues.
Let's say that when you camp, there's a chance of a random encounter. There are two ways to handle this:
A) The encounter happens before you get the rest, so you're still drained of HP and spell slots.
B) The encounter happens after you get the rest, so you're fully refreshed.
In situation A, the random encounter could be downright lethal, and frustrating, because that's the only way to recover spell slots and heal. Players will think "WTF am I supposed to do about that?"
In situation B, you'll be able to start out feeling pretty decent and full-up, but for the encounter to be meaningful, it'll have to be challenging, so it could end up going pretty badly for you, making you feel like you need another long rest right away, which will be annoying. If you handle it easily, it'll still drain some resources and feel a bit like a waste of time.
Because random encounters generally feel like a waste of time anyway.
And I am ignoring the issues of random encounters at camp having an entire camp of people who can't be in the fight at all for balance and gameplay reasons.
I would just implement a fatigue status that gets triggered at important story moments or when companions have something to share to basically tell the player they need to go back to camp. Then have fatigue start a timer that changes to exhausted when it's done, where exhausted would carry some minor debuff. When in a dangerous area or a dungeon have the "go to camp" button instead start a long rest with a chance for interruption by enemies (have the screen go black for a few secs just like a short rest, then either fade in to combat or a successful long rest). Finally put a minimum 1 short rest before going to camp again and a max of 3 short rests. Should structure and limit resting while preventing people from missing out on camp interactions.
There already are rules provided by 5e for the Exhaustion condition, which are debuffs which increasingly stack the more it builds up.
https://roll20.net/compendium/dnd5e/Conditions#toc_15
