Once I finish and publish "Conqueror of the Sun - Severed Galaxy", the most likely following novel to be completed will probably be "Conqueror of the Sun - The Dead Doll Princess".

There are a number of reasons this is the next book: 1) It is the closest to being done novel out of all the other partially completed side book stories (being at 80 pages). 2) The tale takes place almost immediately after "Severed Galaxy". 

The following text will have spoilers:

"The Dead Doll Princess" is mostly focused on Persephone Arc's kids and only takes course over about a year. So, there is a jump in the timeline from Persephone being only 1 year old still or even as mentioned in the epilogue of "Severed Galaxy" at 12 years old when she is married and becomes the Empress of the IRF Empire. Instead, the story starts in the Year 2523 at which point Persephone is 32 and her eldest child is 19 years old. Thus, besides this leap, The Dead Doll Princess essentially takes place right after Severed Galaxy. It certainly is a direct continuation of Severed Galaxy, and I kind of consider them as a mini-saga within the Conqueror of the Sun series.

Below is a depiction of a scene from Conqueror of the Sun - The Dead Doll Princess. The single image itself has a lot about the story, but I will try to summarize as brief as possible.

Closest in the foreground is Jerrold Arc (18) who is trying to protect his little sister, Hazel Arc (12). The twist of it is that she is already dead and has been for a while. Jerrold was able to preserve Hazel in her youthful state from the time of her death. This is accomplished because Jerrold made an unholy pact with Hell. Through some rituals and spells Hazel Arc is not only perfectly preserved but becomes a powerful weapon to use against the Atlonian Confederation.
At first, Hazel is only discovered powers is her ability to communicate AC intel and secrets to the IRF. The next level found is much like the tale in the Bible. Hazel Arc's body is carried into combat via an ark and assures victory for the IRF side. Only instead of the 10 commandants in an ark, in this case, it is a dead little girl.
Hazel Arc's ark is shown in the background of the image and is used to transport her body. This is in contrast to the technological coffin she currently is in for the picture. Since it is embedded into the superstructure of the starship they are on, the traditional ark must still be used to move her.
Being a completely new type of ship, the IRF-ISN Charon uses Hazel Arc's unnatural and extreme powers to make a novel weapon that is able to destroy any vessel that comes within a certain range.