First off, welcome to the site! Great post, it's always exciting to find others who are fascinated by the Halo Universe and lore.
I think what 343 is mainly focusing on since Halo 4 is the Chief's humanity, showing that he can't be the stoic, unfeeling figure Bungie created him as forever. this concept was actually further explored in Mortal Dictata with Naomi as well. Her emotions and beliefs as a Spartan II were tested upon the issue of reuniting with her father. I think we've already started to see some cracks in John's personality and that they use his cracked visor as a metaphor for this i.e. he is breakable. A common belief is that Chief is overworking himself without taking any breaks from missions because he cannot live a resting life and must distract himself from the loss of his closest companion Cortana.
I think adding other playable characters, especially Blue team makes the game all the more special. Following the two sides of the story from the "protagonists'" view and sense of "justice" should make for a fantastic story that will leave the audience sympathizing and routing for both sides.
I think 343 actually did an excellent job of tying the lore from previous and contemporary novels into their Hunt the truth campaign and Halo 5.
As for a prologue game, I don't think I'd be interested in any more war-time stories unless it was a character developing game about a character of upcoming importance or to give more background on an already important character.