I highly recommend creating an actual ForeRunner set based of halo 3. Besides your "Halo Battlescape III" set their is no other sentinel set which I believe to be outrages because of their large role in the halo franchise. Personally I believe a set consisting of Master Chief (Halo 3 style) and the sentinels fighting the flood in either in the Halo mission (Snow environment or the warthog part at the end) or the mission with the long bridge to were the prophet is before he attempts to activate the ring (sorry if this is inaccurate, I forgot the name of the mission). This product will be guaranteed to be successful because of its diversity towards other sets.
Yours truly, Lieutenant Lee