The DLC question was already answered,
I think a sequel is almost guaranteed. Dark Souls seemed to have sold very well for this type of game, and I think it got up to 1,000,000 sold world wide. There was even talk around here of FROM trademarking something called Dark Forest, which seems like a possible title for a follow up. I can't imagine Demon's Souls sold nearly as well as Dark Souls, and we got a sequel, so things look good.
As or the Dragon, it's the hardest enemy in the game, bar none. None of my builds have been able to kill him, but I got pretty close once with a 40/40 end/vit faith character by standing in a spot where his main fire attack missed me. But eventually he just freaks out and starts endlessly spamming a fire attack from the sky that hits everything.
I'm sure if you made a build entirely devoted to killing him you could eventually do it, or if you just overleveled like crazy, you could do it, but he's basically unkillable for 99% of players. Which is kind of awesome.
Althoguh he is extremely difficult with melee, magicn users can get him on the bridge with crystal homing souil mass, and the head gear and ring. I ahve killed the thing each time I have played using ranged magic (soul spear) or simple lightning miracles...I admit I didn't find the "spot" myself and read it in OT 2 or 3, but it is not the one arrow glitch...it is a completely legitimate cheese spot and takes a good 4-5 maxed out INT soul spears or pretty mauch ALL an intermediate level players ligthning, both normal and great, if they are faith based (FAI 30.) I am not sure if faith adds to lightning spear damage or not...but again, bottom line is Dark SOuls offers no enemy that can not be cheesed ion some way...keep looking for the "spot." SIonce pyro does not work as it is out of range this will give you a hint about how far away you are.
spoiler for people who don't wnat to the challenge:
the spot is on the walkway ledge that connects the rats to the underbelly sections underneath the dragon...if you walk to the opposite side of the door way to the rats, turn around, you will have a small vantage point to the HEAD of the dragon. Use your bow to aim first as you will not be able to lock on, then swith to either magic or miracles and fire away!