I saw the Fade spawn animation feature in Alpha and decided needed to go further.
Perhaps this is already implemented by some mod, but I do not know about it.
But here's an idea.

• For example: rendering a mob in 96 blocks.
• When he are in 96 blocks. The mob appears, but has a completely transparent texture, as the player gets closer to the mob the texture transparency increases to 100% on 90 blocks.
• And vice versa, as they are removed.

You can partially do this already, etf has a distance from client property, you could either use progressive more transparent textures over distance with the translucent rendering override or make lower detail models over distance for a lod effect

In fact you can also do this with vanilla shaders