Sounds, quiet ones, for grass stepping, other movements, etc., and npc that hear it within x distance, and move to the most nearby wall to take cover, then peek around the corner to obtain visual, then run at Link. They will also be alerted by sounds from other npcs that aren't within vision and do the same thing to them. Sometimes attacking if a different type of npc.
Random encounters of npc battling each other and changing target to Link if he intervenes.
Sometimes, depending on npc type and/or circumstance, both, all, or some of the npcs react to Link's hostility by attacking him, other times some of them take the opportunity to flee, act as if Link has rescued them and desist violent behavior, or continue attacking their previous target with Link.
Of course Link will be looking for items to loot and battle experience, so he will have to be very selective about what to do. Some npcs are looters themselves, so he's going to prefer to be in the action mostly, but sometimes it might be beneficial to observe and time his participation accordingly. Battle points will not be given for npc vs.npc deaths, and a counter for each npc that keeps track of damage done by Link will diminish the amount of bp Link gets proportionate to that of counter if not 100%,, yet the multiplier for on-screen enemies will remain in effect. Most npcs will never respawn quickly, and some won't at all unless a pair are left alive.
Standard elemental damage/defense stuff will still be implemented, and also have a role in npc-types conflict with each other in some areas, especially where climate change is a factor where hotter enemies meet cold ones that would extinguish their fire, and the fire would melt the ice the cold ones are used too, making either irritated or otherwise in harm's way, and other similar circumstance, like a lava ridden monster encountered by a dry plant.
Of course, some of the npcs will be hunting for food, predator style, and most will have a hunger counter, at least all of the more mobile types that are not fixed in the ground or otherwise fed through some other magical means.
Some "dungeons" are going to be only possible to gain access to their areas with help from npcs, possibly by alternate means upon obtaining skills and items, skills with items, etc. and as the title might suggest, the timeline will reset in the event that the circumstance Link gets himself into prevents him from moving on.
This will also, since this game is not going to be a three day time span dependant on Link saving a town with a terminal condition via Majora, make way for an almost endless amount of possible opportunities for Link to explore involving waiting around with limited npc involvement except for slight level ups after days, weeks, or even years for them to spawn, and maybe even evolve. For example also, a man might appear from it off town on day 5 only if Link doesn't progress through the storyline to x dungeon because he can't yet, but does have the item he obtained from a dungeon he couldn't get to until day 6.