not sure if this is a good idea, but one thing might be a failsafe where all the halo rings attempt to fire, because the ark was destroyed and it might have been the flood. So then halo 4 would be just you and cortana on this forerunner construct, trying to make it to the command room and deactivate the halo's, a bit like halo 1. Only i think this one should be a more machine like planet if it is. Or maybe a two part game? where the first is hijacking a ship from this forerunner planet and using it to get to the halo's, and the second part would be alerting the UNSC and the covenant (the good ones, like the elites and grunts, but not the brutes) about whats happening, and then taking out the halo's
And maybe a few surviving forerunners? IDK just putting ideas out there