I've seen the population trends for H3, Reach and H4. There was significant increase in multiplayer population when North American summer break starts. It's regular as rain.

It seems reasonable that a new multiplayer release would sell well around June 1st. Just in time for summer break and grading presents.

Then campaign would sell as it always does, for the Christmas break. Campaign has been the Halo series strong selling point over the years.

I doubt it will happen but the stars are lining up a bit for something completely new.