Anyhoo... Wonder what other armour abilities they'll reveal... :s Sprinting and cloak are the kind of... "duh" things. Perhaps super duper strength? Super bunny hopping? enhanced armour? (sounds like another game with armour abilities...).
Overshield or bubble shield is another fairly obvious one. Radar jammer would have made sense, but it's been kinda rolled into the cloak (which sounds like a brilliant idea--pros + cons)
An armor ability that perhaps spread a couple of motion mines would be interesting (in a kind of automatic burst pattern). The next time you use the ability the old batch is disarmed. Not sure how it would be balanced. Either tripped only by vehicles (make 'em attach like the beta trip mine, and require a fair number of them to detonate to destroy a warthog at full health, say), or give a fair visual indicator necessitating that you place them in strategic locations, such as below foliage or around the entrance to a building, if they can be triggered by infantry.
Would like it if the beta had Firefight 2.0, scarabs and vehicles. AI Marines or even being able to play as a marine would be interesting enough. They've already proven they can do proper AI scripting if you want to do a truncated, Aliens-esque, Ripley-drop to take out the "queen" and then hop back aboard the ship in H3.... as opposed to driving a mongoose/warthog off a cliff to land on a scarab or... the old-fashioned tank method or... disabling the scarab legs and boarding on-foot...
That was the coolest thing I ever did in the level The Covenant: get on the hornet as a passenger and have the marine fly me over to the scarab. I took out the core, then was able to jump onto the Hornet as it came to pick me up. Sadly, wasn't able to take out the second one, IIRC. Heh.
With the way they've set up Reach as two forces at the very height of their military power, it would be cool to see some kind of super-scarab, say 3x the regular size in every dimension. Fly your team of 3+ coop buddies in on a Falcon whilst dodging several mounted anti-air guns, drop them off, give them air support as they work through. (Pie in the sky of course)
As for Firefight, I'm definitely with you on that, and if you're suggesting they have Scarabs in there (at least for the final game), then doubly so!
Considering they've said there will be multiple modes in the beta, I suspect Firefight will be amongst them.