barren hearth
I think something nice to add would be depending on the distance from the player when an entity appears, it would change how long it takes for it to dissappear, for example if one appears within say maybe below 2 or 3 chunks, once the player sees the entity it would vanish as it does now, within a few frames or so.
If you have a bigger render distance and an entity appears say maybe 6 or 7 chunks away, it would be a good feature to have it stay longer while in view of the player, at max maybe 2 or 3 seconds. My reasoning is that it makes the entity easier to notice, and if you do spot the entity quickly it would be almost as if due to the distance between you two, and if spotted quickly it would be like the entity is staring you down, watching because it knows you can't get close before it vanishes.
I don't know if this would be possible or not and would probably need some testing to get the perfect, bit overall I feel it would be a pretty creepy addition.