Here's a few things I've come up with over the last few weeks.

I (1). Cargo Chaff

I don't know how many of you have seen Star Wars Episode 2, but in it there's a scene where Obi-Wan dumps his fighter's load of spare parts to confuse a pursuing missile.

Wouldn't it be great if you could do that with cargo? Hit "j", and the missile is suddenly faced with a spreading fan of targets. Of course, smarter missiles wouldn't be susceptible to this.

2 (II). Fun with Nanofibers

Now, every space game and its clone has a tractor beam. But what about a nanofiber grapnel or 'lasso'? Fit the handy winch system into a weapon or equipment port, and fling this wire-thin yet incredibly strong cable to haul in cargo, perform impossibly tight turns with the aid of asteroids and stations, or hook onto your enemies to slow them down.

Of course, nanofiber isn't unbreakable. Putting too much stress on the cable (hooking an asteroid at 150m/s, flying in the opposite direction to the player you've hooked, etc.) will snap it, making the weapon unusable. Perhaps different sizes of cable would have different tensile strengths.

3 (III). Build Your Own Capship

This is perhaps the most far-away of my ideas. It's fairly simple:

Take one asteroid.
Attach capital ship engines, warp systems, sensor arrays, etc, using the roid as the superstructure.
Fly.

The thing's gonna maneuver like a pregnant cow, but all that mass is nearly unstopabble. Plus you can hollow the thing out for cheap docking space.

Perhaps you could slowly upgrade the asteroid, reinforcing and reshaping it until it's all ship and no roid.

4 (IV). The End

Enjoy.
-Kwick

I like (1). It would add some interesting situations when you're fighting in an area with bot droppings everywhere. In particular, it should only affect proximity detonation, obviously not contact warheads. This will no doubt bring cries of woe from the missile-philiacs, but it will definitely make things interesting. I was originally envisioning a mine-launcher that served as cache, but cargo could believably react with missiles/rockets.

I really like (2). It would take a great deal of development, but so would many good ideas. Especially if it's used for maneuvering like you mentioned, since the physics engine is only momentum + engine force (in other words, it's pretty simple when you're not colliding with things).

(3) will have to come well after cap ships are introduced, but it would still be cool. The larger roids would require very powerful (expensive) engines, but maybe things could start by just being able to move a valuable-material roid out of hostile space to be mined by collectors. That could be a great group mission: "Bring an xithricite asteroid to sector XX" in a system where those kinds of asteroids are infested with tough bots.

In fact, I would think that the raw materials in a roid would be more valuable as building material for alloys than if used as a ship. Sure, roids are invulnerable right now, but if you make a ship out of it, the strength of the roid will have to be considered, and alloys are usually stronger than ores. So I'd prefer an armored cap ship over a roid ship.

On roid ships;

Perhaps capital ships are produced by 'seeding' roids with complex clusters of nanobots. You may need to move roids into groups to get the appropriate materials for a ship. I.E:

You want a patrol destroyer. You pay out a couple hundred grand or more for the nano kit. You look at the requirements and you see Xithricite ore (1 unit), Azek ore (2 units), and ferric ore (5 units). You scan asteroids to find ones with the required amounts of materials, move them into a cluster in an empty sector, and release the nano package. You'll need to guard the package during transport (can't lose that thing) and guard the ship during the multi-day construction process (can't let enemies spoil it).

An addendum: Keep in mind the fact that a fast-moving asteroid is the mother of all kinetic energy weapons. Just the thing for smashing an enemy station with, if you can get it moving fast enough...