Are there more starcharts available then those on Illium? There is still a huge gap on my map in the right/down quadrant and I wonder if I missed something.

No, those are all that exist.

The lower area of the galaxy includes systems from the first game and is known as "Citadel Space", and is governed by the Citadel. This game takes place in the unruly wild west known as "The Terminus Systems". Don't expect to go to much in the lower areas (other than our solar system and the Citadel)

I didn't think about it that way, it's a sound explanation. 

But, make sure you stop by Uranus and launch a few probes. It's funny. Worth a chuckle, at least.

" But, make sure you stop by Uranus and launch a few probes. It's funny. Worth a chuckle, at least. "

Speaking of the Galaxy Map, wasn't the Citadel more towards the center of the galaxy in the first game's map?  Might just be me.

" Speaking of the Galaxy Map, wasn't the Citadel more towards the center of the galaxy in the first game's map?  Might just be me. "

Well, since they are releasing the hammerhead, I'd imagine that at least some new areas to explore will be released alongside it as DLC.

 
  Sorry couldn't find an image of the ME2 map weirdly, but this is the ME1 map.

Looknig at whiteraven's pic there, it does seem quite a few are missing but it fills the gap for why there is just nothing on the other side. 
 And yeah I think the Serpent Nebula with the Citadel is a bit further to the left-bottom in Mass Effect 2.