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.
You can also stop the spread altogether by placing two coal blocks anywhere within the radius.
The other great feature of this mod is the chance of quartz and nether brick coming through. The chance is low but any opportunity not to be put at risk in the Nether is probably a safe bet.