I'm all for evolving the mode into something with more of an arcadey feel, with credits and scores and such. It still blows my mind that the mode shipped without all that.
I think a Spartan Ops with a hubworld similar to ODST would be great. Each week, a new section of the hubworld is unlocked, where new elements and areas can be explored. Some missions will have you going into the depths of the city, some will have you extracting data from a Forerunner excavation site, and some will have you fighting alongside a starship in a Battlefront 2-style space battle, complete with boarding mechanics and the opportunity to blow up Covenant ships from the inside.
Credits are earned from killing enemies, which can then be spent on improving your SpOps character, as well as improving fortifications of the city, and perhaps of the Infinity. Sounds like an FPSRPG, why the hell not... You can purchase things like score multipliers for Firefight (each location is an area of the city that you can "activate" to begin a few rounds of Firefight, buying extra ammo, lives, upgrades, etc).
The city itself cam be explored, with plenty of easter eggs, and wide open areas for vehicle combat, too. You could have faction-like mechanics, too, where Steitzer or another AI would say something like, "Warning, Gamma sector is under Covenant assault" and have to get your ass over there fast to stop them completely taking over that space. Perhaps this time it's under attack by a Scarab, perhaps next time it's a Liche, or a Harvester, or some dropships. Hell, even that could be reason enough to initiate a Firefight round.
Thinking about it, there wouldn't even really be a point releasing this as a mode for a mainline game. It could be it's own evolving subfranchise that's download-only, constantly evolving and constantly adding content in the Minecraft model, plus DLC, etc.
...yeah