Hmm, I have an idea for a revised Spartan Ops distribution system.
-Week 1 (Prologue) begins with a relatively small, but open vista that marks the beginning of a hub world. In terms of scale, think maybe Paradiso or Sandtrap. Keep in mind that it only has a few small vehicles, strewn about weapons, etc. If it's on a Shield World, it could use a L4D Director system that just throws some basic waves of Sentinels or Engineers at you, whom killing is optional. Alternatively, you could pull a Minecraft and have the hub have a "peaceful" period and every 30 minutes or so there'd be a wave of patrolling Sentinels.
Combat takes a back seat to exploration, with the Hub having things like Skulls, Terminals, Audio Logs, etc. hidden around.
In addition to this, 4 missions are available, sort of like ODST meets Borderlands. They're bite-sized campaign missions, initiated through plot trinket-induced flashbacks just like ODST.
Week 2 expands this hub world and increases the weapons and vehicles available to the player and adds 4 more missions, either continuing the stories of the past week or introducing new arcs (following a consistent set of characters).
This increases exponentially in terms of enemies, firepower, and whatnot until week 10. Week 10's hub is about the size of two Forge Worlds and features plentiful Banshees, Scorpions, etc. and the hub is now teeming with enemy patrols. Maybe even air traffic, Phantoms, a distant Carrier...
Beating the 4 missions of Week 10 essentially "unites" everyone and unlocks either a bonus chapter or Episode that features the entire squad back together fighting their way through the wartorn hub.
Note that hubs 1-10 would have certain "lost forever" elements like an RPG, but on a smaller scale; a Terminal active in Hub 2 may not show up in Hub 6, for example. However, all your material "carries over" to the following hub depending on personal progress, with skulls or data pads ultimately stored in a trophy room, database, or even option menu in the main game.
Dunno if you've seen, but Postmortem from FUD is building out a hypothetical season 2 with concept art and level designs, might pique your interest since you're thinking in similar spheres. http://www.forwarduntodawn.com/