We've got inter-spatial travel (where there would be minimal gravity to consider; so aero and hydrodynamics are of little to no concern (unless there's an off-chance you crash into a space ocean made up of condensed dark matter, compressed to the degree it appears as water to us. Ultra-dense; not even a ship the size of the moon would last too long in something like that.)), Orbital combat (watch out for the satellites; a space elevator would be a primary means of bringing forces to and from surface), and sub-orbital combat (equivalent of aero-naval combat/Airship battles; some can go orbital when needed (you can tell them apart from orbital/inter-spatial ships due to their aerodynamic forms; some being hydrodynamic as well for hiding beneath the oceans.). Airship combat at orbital levels would be a hybrid of dogfighting and naval combat, complete with sub-atmo and exo-fighters capable of spatial combat from carriers that would be looming just at the ceiling of atmospheric and gravity's influence (hell-dropping fighters into aerial combat)).
Of course, if we're going system-to-system combat, then FTL combat would need to be considered, especially if you expect to defend the oort cloud inter-spatial barrier/border (keep illegal aliens out of our star system).
There's alot to consider here.
If there's one thing that's consistent, it's that anything can be destroyed if you hit it hard enough. If you have the tech available to, I would invest in kinetic weapons, and a sort of space-brake counter-inertia system (or inertial dampener) to counteract the 3rd law from sending you to another star system upon firing (depending on the size of the cannon used, exaggerated as it sounded; but then that would also mean you'd have to build a rather shock-resistant ship to handle the outgoing force, as well as the resistance to said forces; an accordion-like ship or cannon may need to be designed to handle all the forces considered.). Alternatively, you can always make pretty big railguns. Who needs energy weapons, when you can send slugs of metal at ships at such speeds, the target ships would explode from sudden decompression (although with energy weapons, you don't need to worry as much about gravity influences as much, and it's easier to snipe with light rather than solid matte; though even light bends around things; just less to worry about where light is concerned.)? You can also hurl meteors/asteroids at your targets as well. Just manipulate the gravity wells of the asteroid enough to aim it, and then have enough leading mass to launch it and guide it to your target at a quick enough speed, that even if they had time to react, they wouldn't have enough time to manipulate the oncoming meteor out of the way.
EDIT:
Oh, right; although the chances would be unlikely, there would have to be forces and etc. that would have to defend above and below the elliptical, where our planetary horizon lies (orbits). Those make excellent hiding spots, and are likely to be open space, ripe for combat skirmishes all over the place. Plenty of space between the elliptical and the oort cloud, as long as you're above or below the horizon of it.