Personally I think there should be a fatigue system. Time based, but also healing based. Time passes and fatigue increases. Healing also increases a character's fatigue. Resting resets fatigue. If you are out of short rests you would need a long rest to reset. Otherwise you are travelling exhausted with lower move speed and stats. Also likely need some cooldown or something on long resting.
This is a good idea, currently there is literally no downside to me not resting apart from simply not regaining spell slots, but it takes me ages to actually use up any spell slots anyway because cantrips are so OP in this game. Literally the only reason I actually have to rest right now is the fact that Lae'zel keeps saying she needs a nap, the big baby. We've only been awake for like 5 days now, suck it up!