Well, to start with you should look up details on the location. You have not stated you have. Eastern Fringe On lexicanum
Two seperate Tyranid invasions: Behemoth and Kraken, came from there.
As does one Dynasty of the Necrons: Sautekh.
The entirety of the Tau are located in that area. (Damocles Gulf Sector)
No information on the planets that i can find so you are relatively free to make them up.
Some passing mention will have to be made to the various Alien and Human settlements that are located there.
Look at the descriptions of the other Segmentums for idea's and layout type.