I think for some purposes the old Hudson Bay or Trade musket was the idea firearm for hunting and living off the land. But it could be improved using modern materials. The ignition to remain as flint strike or something similar to it.
Either a stainless-steel barrel or one that is chromium plated.
Must resist corrosion when the gun is not cleaned. Also use the best metallurgy to allow usage of any powder that one happened to have on hand. Such gun could be used with steel irregular projectiles if needed.
Coil springs of modern steel for the lock and robust lock mechanism.
A synthetic stock.
What other things could be done relative to materials to improve it?