I would like to see something new, not just some new enemies and a couple of new objects and a new vehicle, but actually something new. Also i would like to see more choice.

*SPOILERS, maybe*

The helicopter ride, which i assume will be in there should be interesting and i would like to be able to choose whether I can pilot it or gun it assuming there will be some sort of dog fight as the combine are trying to protect some top secret stuff over there.

Also i would like some light shed on the Gman, although knowing Valve they will milk this for all it's worth and will probably keep it a mystery until HL3 and beyond.

Some new weapons would be nice, or at least having them modified in some way or another. I would like to see more of those new fancy physics (such as the train bridge collapsing at the beggining of ep2) perhaps the player would be able to cause such things. I would like levels to be bigger and not so linear, at the end of HL2 i was hoping i would be able to walk around a crumbling City 17 in a GTA kind of way. In the end of ep2 i liked it, with the big battles, that was fun; want to see more of that.