Have you looked at Orion's arm? Very nice stuff.
One large category of possible engineering is gravity based engineering. Not (necessarily) anti-gravity and such, Engineering on a large enough scale that gravity becomes a factor, once you step away from the two body problem with circular masses many interesting things become possible:
For example
this, it's unstable but that just needs a small amount of stason keeping, what use this would have is unclear but you can see the potential. Another idea is some sort of gravity slingshot, perhaps for long-term high throughput interplanetary transport you could put a few moons in a complex orbit perfect for gravity assists. Even without the tools for massive restructuring of the celestial spheres you can still get in on this, for exsample rotating skyhooks, a cheaper alternative to space elevators. You put a small captured astoroid on an elliptical orbit with the pereapsis just above the atmosphere. As it reaches pereapsis it drops a very long cable with a hook on the end into the atmosphere where a very fast plane attaches cargo to it, because the cargo is slower than the astoroid it swings back, up, and around, by the time it's above the astoroid it's going faster than the astoroid by the same speed it was previously going slower and is now well out the atmosphere, then it can just disconnect and go on it's journey, whatever that might be. The astoroid can be resupplyed the velocity it lost at lesure and thus can use high efficiency engines like ion engines.
Another even more immense feild that's ignored is tech that works on different principals but there's already some other way of doing the same thing, for exsample you want to mine on the dark side of a planet in a close orbit around a neutron star, anything conductive would melt almost instantly due to the massive electromagnetic flux constantly being generated by the star, in that case the miners might make there tech based on fluidics and pumatics or photonic computing instead of electronics (ever looked at fluidic circuits?). Or maybe we get stuck technology wise and a colony desides to redo science and technology along completely different principles to see if they learn anything new that we missed.
Edit: Another neat orbital dynamics
thing.