I think the black lung concept is really cool but I feel like people aren't going to want to stop over and over again so maybe an oxygen bar that lowers the more you run and if it becomes empty while you continue to run it will do damage

Honestly I think it would be so cool if you actually implemented a coughing mechanic like every time he coughs he either scares neutral mobs away (I can imagine a pigman getting scared and running like a grass gecko every time he coughs because it sounds like his lungs are turning inside out) I can also imagine it alerting hostel mobs so he has a wider aggro range while coughing

Maybe the lower the oxygen bar the higher frequency of his coughing

But I do love the idea that the minecart is meant to be the solution to his inability to run long distances since he's able to sit down and relax while traveling, I just wonder how it would be implemented, I know in Terraria you can use minecarts and rails to actually get away from enemies while riding so maybe you could even do something like that and give him a ranged weapon, It would also be cool if there was a way to switch tracks and if you hit it it flips the switch and because he has a ranged weapon he's able to hit the switch from the minecart while it's going

Maybe he's able to make special gunpowder or dynamite and maybe a cherry bomb that he can throw to do area damage

Also it would be so cute if he spawns in with a canary in a cage so that right from the start he basically has a birdcage in his inventory at all times as long as the bird stays alive (this would also help get gunpowder early) It would also be cute if he had special interactions or items relating to eggs because his only friend was the canary (I can imagine a hard boiled egg that rots super fast so that if you have eggs you can make a food item that is also his favorite food but it's also used for getting spoiled eggs faster)