At the end of the slow-motion video, it showed the Ark, then a Halo charging up and firing, perhaps the end of Halo 3, then after all of that, it flashes the "symbol"
Perhaps it's the symbol of the creaters of the Halo? Perhaps they're coming back. Maybe we've been lied to this whole time, and that the ancient beings that created the Halo, hadn't died off.
My theory seems plausible. Forerunners were so technologically advanced, they crossed galaxies speeding up the evolution of species, or even creating them. They eventually created the Gravemind, and seeing as it was near-perfect, it could withstand almost anything, including time, (hence it's age.) the Forerunners began to see the Gravemind their greatest creation, seeing the Gravemind as the highest sentient being, other than themselves.
Then the Forerunners created us Humans, and seeing as the Forerunners so closely resemble us, they saw us higher than the Gravemind. The forerunners told their other sentient creations, (including the Gravemind) to regard us, (humans,) as the highest sentient being, other than themselves. We were loyal, and the Forerunners began to bring us into their soceity, and regard us as equals.
The Gravemind saw this, and even when the Forerunners regarded him so highly, they still did not accept him as an individual in their society, so he rebelled. He created his own species. He knew all of the Forerunners secrets hence they created him, and he spent thousands of years with them. The species he created were perfect to bring down the Forerunners. Their strengths were the Forerunner weaknesses. They were known as the flood.
(The gravemind looks like a flood, green tentacles... and if the Forerunner made us humans look like them, the gravemind created a race to look like him.) The actual "flood" was just an infection that the Gravemind seeped into the Forerunners food supplies.
The infection turned into a suffocating germ, which floods the lungs. When the patient is brain-dead, the germ slowly adapts to control the body. It grows tentacles, (mini Graveminds!
) and spreads them into the arms and legs. The bodies that the flood vehicle are actually dead, decaying, but the flood controls it kinda like a puppet on a string.
When the gravemind was regarded so highly by the Forerunners, they fed him their bodies, so their knowledge would never be lost. Because of this, he became extremely intelligent, and the Forerunners allowed him to operate many vital things to the Forerunners, Food, liquids, and other things.
Eventually, the Halo 'worlds' where the Forerunners and Humans lived peacefully, were turned into military fortresses. In a last ditch effort, they created the covenant as their military species, but it was too late.
Fearing everything was lost, and that the Humans and Covenant would eventually find each other and kill each other, the Forerunners commited the ultimate sacrifice.
They gifted us humans with their genes, and intelligence, and the Covenant with what spare technology they had.
The forerunners sent what was left of us Humans to earth, wiped our memories, and bed us farewell. Because they loved us, they created a super-structure to defend us against the Flood, and covenant when they came. (hence the Halo 3 trailer.) The left. Then they brang the covenant to their homeworld, and bed them farewell. With barely enough time to spare, each ring was simultaneously set to operate, and it obliterated the flood, except some samples kept to study.
Being the worry-bugs they were, they were concerned that the rings wouldn't be enough, and so they had to study them, and find a cure or defence. It failed, and the lab-specemines brokeout, and now roam the Halo's.
Will take too long to spell-check. Thanks.