I sincerely hope Firefight returns. It was a great gamemode. However, if it does, it needs some serious improvements. Halo 3: ODST had the feeling right: desperate, on the brink of death action. Players had to conserve ammo and scrounge for weapons to stay alive. They had to work together to sustain their lives pool. It was damn near perfect.
Reach introduced customization, and while that was fun, the spirit of Firefight seemed lost (in my opinion of course). Sets were even more repetitive because new skulls didn't activate at the beginning of each set. Ammo and power-weapons were plentiful and seems almost infinite at times (they weren't, but it took a long--Yoink- time to run out, assuming you don't turn on infinite ammo or bottomless clip).
If 343 can bring the feeling of ODST together with Reach's potential for customization, Firefight could be a great gamemode again. Maybe take some ideas from Infinity Slayer where doing well and surviving earn new power weapons and/or abilities (in addition to lives). Takes some ques from other survival modes like Horde 2.0, and allow players to fortify as the games goes on.
Firefight made a big splash when ODST released, in part because it was unique among survival modes. Let's see if we can recapture that feeling!