Exploding barrels (for those with ranged weapons) - could be two kinds, one filled with Alien acid, and one filled with Human explosives
Destructible crates (for those with melee weapons) - could be several sizes, with larger ones being harder to destroy
Exploding barrels: Hrm, this seems to favor Humans, but maybe they should be in the default Human bases in maps where they are used to balance this? Very few Alien classes would be able to detonate them from afar (+goon, +mara?, +granger?, maybe 1.1 rants) without dying, but barrel jumping might be cool. This idea rates a "Hrm".

(Sounds cool, could easily get annoying, seems unbalanced, maybe the lid could provide a safe haven for the Alien detonator, or maybe dretches could use them for suicide runs inside Human base, what would their "hit points" be?).
Destructible crates: Seems useless, but if foe were hiding behind a nest of destructible crates, you could slash/shoot your way in. Also, some areas of map would be made inaccessible to certain classes, if, say, a large pile of crates were destroyed (although it would suck if that were the only way to play a certain map, it could add enough depth to somewhat counter the Human advantage - since they have mostly ranged weapons - if Exploding Barrels are added; since Humans would be more affected than Aliens by accessibility issues). Could you push crates around with this engine? You can sure touch and manipulate a lot of artifacts in RtCW, so maybe. Would you want to? It might make the game a little too Deus Ex-like. "Hey, push that crate over here so I can climb up into that open air-vent!"
I like the idea of Human barrels in Human areas (like a grenade going off), Alien barrels in Alien areas (acid splash damage), and destructible crates of various sizes in neutral areas (default, that is, at mapstart). But maybe only if put there by the mapper, or as a layout/devmap kind of tweak for interested server ops/admins. I wouldn't like to see them spread willy-nilly over the existing maps.