The only way to increase the fade margin would be to increase the size of the antenna. Consider that the present dish is 3 ft diameter (about), a 6 ft antenna will increase the gain by 6 db, a 12 foot diameter antenna would increase the gain by 12 db.
The present system will have about a 6 to maybe 10 db fade margin. If you changed the antenna to 12 ft (kinda big) you could have about 18 to 22 db fade margin. A heavy rain storm can produce 30+ db fades at Ka frequencies (the new stuff). Thus, even if you went to a very large antenna, it would change the rain fade very little.
I have both 6 ft and 10 ft satellite antennas in by back yard. A couple of years ago I modified the 6 ft to accept a dual LNB for Directv and connected that LNB to a couple of receivers. I also have a 3 LNB Directv antenna with other receivers connected to it. During heavy thunderstorms
the receivers on the 6 ft dish would have signal about 20 seconds longer and return from fade about 20 seconds quicker than the ones connected to the 'normal' antenna.