Nether Portal Spread is a neat aesthetic mod that makes Nether Portals a bit more ominous. It allows Nether blocks to start replacing blocks within a certain radius and gives the feeling that they are unstable anomalies in your world.
As soon as the portal is open blocks will start changing. Currently dirt, grass, stone, sand and gravel are only affected. Over time the spread will continue in a 15 block radius but that number can be increased to 100 in the config file.