FF XII is going to be based on The Struggle For Empire by Robert William Cole, a 1900 novel in which the Anglo-Saxon Empire of Earth takes on the might of the empire of Sirius. It has starships (using ether propellers), death rays of various sorts, antigravity, and a VERY high body count.
Review of it here:http://airminded.org/2009/03/16/the-struggle-for-empire/
The trouble is that it's only about 30k words and characterisation and background are a little weak. Most notably, his alien worlds just aren't alien, they're earth with rings or whatever. So you have the inhabitants of Jupiter and Saturn watching the progress of the war as wrecked ships crash down onto their towns etc.
I'm not convinced that even agressive terraforming by alien space bats can explain that one, so I'm thinking in terms of using the technology and basic empire versus empire setting, but making the worlds more alien, with e.g. cloud cities high in the atmosphere of Jupiter, or colonised moons a la Weinbaum, not colonies on the surface. Basically using the book as a starting point for the setting, rather than taking it literally.
So I'm looking for alternative descriptions of the solar system - the one that comes to mind is to re-use George Griffith's Stories of Other Worlds, which were the background for FF II, and if possible retain loose compatibility with that while taking the story a couple of centuries into the future. They're athttp://www.forgottenfutures.com/game/ff2
The version of these stories in A Honeymoon in Space (also accessible there) has a lot in common with Cole's setting, such as a big war at the start.
I might also take in some other fiction such as On The Martian Way by Capt. H. G. Bishophttp://www.forgottenfutures.co.uk/mar_way.htm
Any other suggestions on copyright-expired material to support this?