This is just speculation but it seems to be the most probable one so here we go:
A Halo Wars 2 trailer with gameplay
Halo tv series update?
Some Halo 5 announcements
A new trailer for Halo 5 to tell everyone that the game is complete
A trailer for Forge-Halo 5 Guardians for windows 10
And it will all end with a teaser trailer for Halo 6