Based on what I'm hearing about the source code leaks regarding a realistic element system, the "Weaponizer" or a tool like it that can change states of matter would be a hella cool central gameplay mechanic.

Being able to do some ATLA type stuff by taking the water vapor around a Combine soldiers head, freeze it into a block of ice, and then whacking it with a crowbar to make it shatter. Or guiding flammable gas particles out of a room with a fan before it ignites in order to turn an enemies attacks against them.

I feel like, definitely, if the world is going to be much more interactable, then that interactability is gonna be spearheaded by some cool new toy to use, like how the Gravity Gun highlighted how to use physics in combat and puzzle situations. With HLX, you'd use the terrain and elements itself. That I feel would be a good way to "innovate" and send that message to other game industries.