Gamespot Rumor Control has a source that claims the next Halo title will be tactical, "a cross between Ghost Recon and Gears of War" and will be announced sometime this year (maybe tomorrow?). According to the source, the "darker, grittier" iteration is more akin to Neil Blomkamp's shorts. Oh yeah, and that Master Chief guy is nowhere to be found.
In terms of gameplay, you can reportedly play as an entire squad (huh?) in the entire campaign, and there will be online co-op. The mention of Ghost Recon is particularly interesting, as a former creative director from G.R.A.W. developer Red Storm joined Bungie in August 2007. Gamespot seems confident with their source, and that would make for quite a big E3 announcement.