Map size is a large concern of mine at the moment. It feels claustrophobic, without much room to maneuver.
Just wanted to say that battles take place in a separate space from travel. Some of the combat maps are already fairly large, and take a *long* time to traverse in a battleship. "Turning the Tables" and "Sinking the Bis'mar" are on the small side, though. In the full game, you can expect battlefield side to correlate to the size of the opposing fleets - if they are bigger, they'll get more room to maneuver.
As far as terrain, I have to be honest that it's not a priority, given everything else there is to work on. These are cool ideas, and you can expect to see some of those trickle in over time, though.
And btw, coronas are already in the game
(there's a star in "Turning the Tables", though it's admittedly small) You can also mod in a super-dense asteroid field if you like, and change the asteroid speed for much-increased damage (actually, easy enough to tweak any existing asteroid field - iirc, the parameters are documented in the comments).
ooh, on the asteroid one, given that the whole asteroid mechanic is implemented in a more stereotypical way than a realistic way anyways, I think that you could have a lot of fun with heavy asteroid fields.
I've got half a mind to try huge (say, Onslaught-sized) asteroids. Seems like those would be a lot of fun.