Yes. In fact one of the reason for sectors was to introduce terrain. You can think of them as connected games of GalCiv, or you can think of them as continents dseperated by vast oceans. So from the perspective of blocked terrain, we have it. Lots of it on the large scale.
And not only does it blind you to whats on the other side, but you are unaware that a path exists until you get the correct tech. Is that stream into the heart of your empire, or are you going to need to plow through the Drengin territory to get to it? As well as exposing you to the surprise ships you mentioned when they come in from a stream you either hadn't discovered or knew was there, but didn't know what was on the other side.
Of course it also introduces choke points, and areas that are very valueable to control for diplomacy and trade.
But it is pretty macro scale. Within a sector you are right. We do have blocking terran now, though its pretty minimal (black holes). As Draginol mentioned, it could be cool to have mbulas that applied this on a larger scale. Or at least applied it for most factions.
I would love to see a better integration of some of the hostile entities and nebula. So Pirates or space monsters always have their star bases in them. Something to make players nervous when they see nebula and give them a little more personality.
Messing with visibility is an interesting idea. But in a game with the scale of GC4 whenever we tinker with "surprise" on the mico level it doesn't end up being fun because it forces the players to hyper focus on everything. There is just to much going on. Having your fleet destroyed by an enemy you didn't know was there is not a good gaming moment. It works okay if oyu are managing 3-5 units. But when you have 30-50, it falls apart and feels unfairly punishing. We want the player to see the board, want him to recognize the htreat and want to give him time to deal with it.
Thats my 2 cents. I'd be curious to hear your thoughts.
along with lots of other people it seems, i agree that bigger nebula would add another fun level to the game. i also agree that you shouldn't be able to scan past them much, maybe just see inside the first visible tile or something. i think that there are a few things you could do to help negate the side effects of having them be too powerful for sneak attacks though...
1. prevent ships in nebula from scanning anything around them past 1 tile. a ship hidden in a nebula should need coordination from something outside the nebula to be able to see when a target for sneak attack is available. it might also help let you know that there is a possible threat if you see a starbase or ship with high sensors right next to a nebula
2. greatly reduce the logistics ability of any fleet in the nebula down to just 1 ship at a time, and disable the ability to have multiple ships stacked on the same tile. this will prevent giant fleets from sitting in one spot and jumping out at once, instead you will have to spread them all out, which makes sense considering it's a lot harder to see the ships around you in a nebula. this would also make smaller ships less dangerous than bigger ships in nebula, and ships would have to possibly waste more movement if they tried to group up into a fleet after jumping out of the nebula.
3. keep the movement speed reduced for the entire turn that ship has touched a nebula, or maybe a set number of tiles after it leaves the nebula. think of it's as the nebula's slightly lasting effect on engines that travel through them and suck up that nebula material. this will prevent ships that jump out of nebulas from quickly reaching a distant target. also, combined with the "1 ship per tile in a nebula" thing, this will make it take even more time and movement to group up a fleet that jumps out to attack a closer target as well.
4. this might make abilities that let u disregard nebula abilities too good, maybe just make them have reduced effects instead if you're making the nebulas a bigger thing.
5. if you could see into the first tile of a nebula, but ships in the first tile of a nebula still couldn't see past 1 tile, it would help prevent against sneak attacks a little more.
i think the larger nebula idea is also a great way for players to get their ships like colony, constructor, and probes, to areas that would otherwise be difficult to get to without detection, but at a price. maybe even have some research tech to make it so u can hide bigger fleets in nebula and navigate them better for races that are a little more "sneaky" 
