In this hypothetical scenario, the Reapers used the flood to wipe out the Forerunners for some strange reason however when Halo was activated at least 1,000 Forerunner survived and managed to find a Precursor device which would allow them to relocate, soon after they rebuild in the Star Wars Galaxy, the Reapers after finding out use the same device to get outside of the Star Wars Galaxy, both prepare for the worst as the Forerunner stick to several areas and start creating Installations including a new Ark which is to create new Halo's, soon after the Reapers join the CIS to do the usual and to eventually stab them in the back thus bringing the Forerunner into the conflict.
The Forerunner are half of what their original empire was but have almost perfectly remastered their original Stuff, they only have one Halo so far. The reapers have Mass Relays in the new galaxy as well and are almost as strong in numbers as their ME versions. Both star wars governments are full power.
No blood lust, common sense and no PIS and CIS. Who wins this war and what happens afterwards?