Real answer: Basically anything you can think of involving military satellites. Military satellites are already vital for any kind of communications, intel, surveillance, etc. And they will be targeted with all kinds of attacks, both physical and and non-physical, the next time the US or NATO has to fight a near-peer foe.

We create defensive systems for those satellites, use satellite imagery for military intel purposes, maintain their orbits to keep them both protected and effective, work with the big mil contractors to develop new space flight/launch technology, track space debris to keep military and civilian satellites safe, develop network/computer technology to improve satellite communications and security... it's a much bigger and more important domain than most people realize.

One recent example: remember when Russia recklessly blew up a satellite last year and the orbital debris cloud threatened the ISS and the astronauts on it? The Space Force was called in with NASA to help chart the debris and calculate the maneuvers needed for the ISS to avoid it.

Yes, there are always spots open. Enlisted, Officer, and civilian/contractor.

Source: I'm in the Space Force