Because of the (apparent) failure of Spartan Ops, I think Firefight could (and should) return, with more customization than ever. The default mode should be as hard as ODST Firefight. Firefight voices will also return, and I got some ideas for it.
Firefight Voices:
(Original Trilogy)
John 117- 10,000 CR, Exclusive to Spartans
Arbiter- 10,000 CR, Exclusive to Elites
Cortana (AI)- 10,000 CR
Sargent Johnson- 10,000 CR, Exclusive to Spartans
Rtas Vadum- 10,000 CR, Exclusive to Elites
343 Guilty Spark (AI)- 10,000 Credits
(Reclaimer Trilogy)
Commander Lasky- 10,000 CR, Exclusive to Spartans
Gabriel Thorne- 10,000 CR, Exclusive to Spartans
Sarah Palmer- 10,000 CR, Exclusive to Spartans
(Rooster Teeth)
Epsilon (AI)- Find all RvB easter eggs.
And more…
I think credits should return, but separate from rank. The higher your rank (which should be based on skill, not veterancy/ amount of time played), the more credits you earn after games.