Over at HaloFanForLife (and other places), it is stated that apparently there is a voice-over easter egg in the Halo 5: Guardians Beta Trailer, as said by Frank O'Connor (which I can't retrieve the original post by him now).
At around 10 seconds in the trailer, a Spartan yells, "Ready? Let's move!" and the shenanigans continue afterwards... but that initial voice-over is speculated to be Nathon Fillion, aka Buck from ODST and Reach.
Due to his long career in the Human-Covenant War, it would make sense to be chosen by Jun (I'm not sure who chooses the S-IVs) for his extensive service record. As the saying goes, "If he were any better, he’d be a Spartan”.
To further reinforce this Nathon was attending E3 this year, but he could just be doing a double wammy for Destiny and Halo (since he's a big Halo fan). He's seen here monkeying around with Dan Ayoub of 343i:
Watcha think? Yay, or Nay for a side of Buck in Halo 5?
Source: http://halofanforlife.com/?p=15255