Though we've yet to see 343's new Halo trilogy, it seems like Microsoft's fairly certain of its success. In fact, Microsoft's confident enough to already be planning another trio of Halo titles down the line.
Recently, Microsoft's been spotted registering domains for Halo 7, 8, and 9. While that's hardly a confirmation of another trilogy—it's still way too early to say—it's obvious that further sequels have crossed the minds of someone at Microsoft, enough for the company to want to cover its tracks by buying up domain names.
Then again, it's also possible that Microsoft may just be keeping domain squatters at bay, lest anyone attempt to profit off of a domain name for games that won't see the light of day for at least another half a decade.