I know that 343 probaly have the episodes planned out but what if we got to plan the episodes instead of 343. My idea would fit in around episode 9 and lead to Halo 5 (if the Flood return).
Episode 9
So my idea is your on Vortex. An army of Prometheans and Covenant are gaurding a Forerunner base and you have to infiltrate it and get the Covenant battle plans, also take some Forerunner tech, that will be a huge threat to the UNSC. After killing off the Covies and Prometheans and extracting the battle plans a cruiser is heading to your fireteam to glass the area. Infinity fails to stop it and the cruiser fires.
Episode 10
You fireteam wakes up to a battle, however it’s not UNSC that is fighting the Covenant. It’s the Flood. The Flood was in a research sector not to far from the area that was glassed. The debris from the area hit the buildings and the Flood was released. As the battle continues Crimson learns that the Flood here have evoled to a much stronger state than the infestation found on the Halo Array. Palmer calls in for a pelican however no pelicans can get near the sector as a fleet of Covenant ships have arrived to destroy the Flood. Drastic measures are made and a group of drop pods are sent down. In them is Blue Team and Spartan-117. After a huge fight the battle ends and a pelican comes in and gets the spartans. However the battle is far from over now that the Flood have return and are much more dangerous. Back on Infinity a surpise appearance by a certain elite makes the crew have hope they can win. When the elite hears of the UNSC’s "simple " plan to stop the Flood and the Covenant the first thing he say is “Were it so easy.”