If you want to challenge yourself, you could try taking planets faster instead of just increasing the number of neutral frigates. For instance, try getting 5 planets by 8 minutes. If you give yourself a speed requirement, it's definitely a difficulty increase. 5 by 8 sometimes isn't possible, but 5 by 9 or 10 normally is.
Not saying that this is something you should do, but it's an interesting and challenging exercise... and is very useful practice for competitive, reasonably spacious maps.
