I would like to bring to light my idea for something that, if no one has realised already, a race that can really fill in the niche thats left with the main ideas only including Covenant and UNSC. I believe the 3rd race, which could be placed as the Advent (or the Verdani, which would fit into place story wise as the race that is in a way threatened by the flood), would be the Forerunner Monitors, the machines left by the Forerunners to safeguard the 7 Halo Installations as well as the Ark. There is a Flying unit from Halo 2 that could fill the role of the fighters, its the big ones you face with the shields n the minicannons, they were like drone fighters in relation to ship size. Now i havent played the game Sins yet, but im guessing you have governors of planets and such governmental systems? So for the Machines the Monitors can be your governors. Another gameplay feature for this race could be that they start off with The Ark homeplanet, and you have to manufacture the 7 rings, the superweapon of the race then being "wipe out all life in a certain (large) radius around the ring" which can be activated by all 7. Possible cap for number of rings, limiting them to 7., Their ships could have a general design of the mini laser defenders, perhaps the main capital ship being one like an Ori Battleship from stargate, where it has the massive continuous laser (taken from mini defender). What else could there be...oh there was a map both in the halo 2 campain and in multiplayer where there is a gas harvesting facility, and it looked like an addon to the ring, attached by a massive electric cable? I think good idea for Logistic structure. Also, with the whole thing of space junk in the original Sin game, when a Ring is...eventually...destroyed, it leaves a huge space junk area.
Just an idea to fill in the "3rd race gap"
Dubaian.
<-/-()-\->Some say his voice can only be heard by cats and that hes banned from the city of Chichester. All we know is hes called the Stig<-/-()-\->