Not sure if her appearance would count as a lore fan mind-blower. While I would prefer she stay dead, she will most likely return sooner or later.

: Joseph Staten as Grunts

This one would certainly qualify, though it has more to do with the games than the deep lore.

: Buck

We heard him in the MP teaser already, a lot of people expect him to appear as part of Locke's team, and again he's familiar to people who aren't that versed in the EU fiction.

: Blue Team

Everyone who's been paying attention knows they'll appear already so they wouldn't count as a mind-blowing reveal.

: Halsey

Not too mind-blowing either as she's a mainstay of the games by now.

: Iso-Didact

This one would definitely qualify

: Medicant Bias

Ditto.

: Gravemind

Possible, but he's pretty well-known to game-only fans too.

: Mendez

A strong candidate.

: A Precursor

Possible

: Offensive Bias

Falls in the same category as MB, but is more obscure and less important of a character so Mendicant is the more likely one of the two. Unless Offensive is with the "good", formerly exiled Forerunners who I suspect to be the "Guardians" from the title.

: Johnson

I think we can safely rule him out.