Oups, sorry, I didn't see that Willempie already recommended the Grunfeld.
Anyhow, this is beyond me why someone would recommend him to play the Slav: of course, the Salv is a great opening, very trendy, but it is soooooo different from the Dragon that I can't see the point.
If I were to recommend a closed opening, it would be the semi-slav which usuallu leads to very complicated games, one could even say wild crazy games in the botvinnik. But there are many dull lines if white wishes so.
I don't really see the appeal of the Benko either. It's all about queenside pressure, and not very varied. The big advantage is that it is easy to grasp. Well, relatively easy of course...
But it's not very "dynamic" in my opinion, and you are never going to attack the king if that's what you like. Never.
This leaves a choice between the Leningrad, the KID, the Benoni and the Grunfeld.
The KID is very complicated and actually very different from the Dragon since the centre tends to be very closed. The only similarity is of course the dragon bishop and that you sometimes get to attack the king. There are also some opposite-side castling variations, in the Saemisch for instance.
The Benoni is a good choice I think, but it is very risky positionnaly speaking and hard to play. Additionnaly, you can't force a benoni.
As to the Grunfeld: there is also the drawback that you can't force a Grunfeld, but I think most of the sidelines tend to give rise to a rather similar game to the main lines, i.e. fluid center, open lines and diagonals...
The Grunfeld it very dynamic, i.e. see the games of Sutovsky and Svidler although it tends to be a rather diffeent kind of dynamism than in the Dragon. Of course, it can also be argued that lots of line lead to queenless middlegames which might not be to everyone's taste.
One advantage of the Grunfeld is that there is an abundant literature, and arguably the best opening book ever written, i.e. "Understanding the Grunfeld" by Jonathan Rowson. There is also a forthcoming book by Dembo which is going to be, I hope, up-to-date. There are also very thorough books by Sakaev and Dearing on the 7. Bc4 and 8. Rb1 lines respectively.
Hope this helps