I think for capture additional mechanisms aren't needed to reward using larger fleets because if you just send a scout to take a planet your enemy just has to kill that scout to take your planet away again. Conversely you can use a small strike force to take a planet in the enemy's rear and force him to get some stuff moving back there to take it back or you could even use that planet as a warp location for a large ship.
My only quibble with this is that I think players will tend to have a really negative reaction, if it happens instantly. It will allow people to win games against newbs by just luring their fleet into a battle of attrition, then taking all their rear planets. I don't think people will like that, if they don't have any time to react to it.
Sensors are difficult with Spring's handling of radar and LOS, I want to avoid using radar because I really hate that inaccuracy. Hm, maybe a gravidar that tells you the total mass located in a certain area?
It's a hard issue, to be sure, since it's tied into so much else in the game engine. I was thinking that maybe you could have a LUA'd ECM / ECCM system, that if one side "burns through the jamming", changes the sight radii of that side's Units, in that specific area. That'd be a royal pain to implement, though.
Not sure about blocking stuff, I don't like the idea too much.
I wasn't thinking so much of blocking, in the traditional sense, as of random casualty generation- i.e., you can fly through the asteroids, but you may lose a ship or three out've a fleet of 20. Anyhow, that's just a random thought on this, I think that surprise, both tactical and strategic is such a big deal for a game like this that anything that makes it more interesting may be worth pursuing.
Burn? Like Maelstrom's "move faster" option?
Pretty much, yes- just a simple bit of LUA, tied to a "burn" button, that changes a Unit's top speed and acceleration for a number of seconds. Then you'd have another balance tool to use, too- some ships might recover from using it faster than others. Gotta find ways to make up for the lack of terrain...