What is on Red Line? At one of the intersections of Grand Line and Red Line are both Mariejois and the Fishmen Island. I believe it is the only way to go to the "New World". Since we don't know much about it, I was wondering what kind of secrets it could hide.

What exactly is Red Line and what is on it?
The Red Line is basically a group of islands connected together.

We know that it has it's own season islands, as explained by Nami


  The Red Line that crosses the Grand Line has its own Summer, Winter, Spring and Fall "islands" chapter 604


It is 10,000 meters deep, bears the world capital Mariejois, as stated by Hachi.


  The Red Line bears the world capital, Mariejois, and is said to be 10,000 meters deep from ocean level to Fishman Island chapter 496


Due to the same height, it is also considered impassable, all though this has been proven to not be the case as 


Fisher Tigre Managed to climb the redline to attack Mariejois as told by Boa Hancock in chapter 521 
Germa66 passed it with an amphibious ship in chapter 841


Besides the sheer height, it is also considered indestructible, as demonstrated by Laboon in chapter 104, as he had been ramming the red line to no avail for a considerable amount of time.