Well I guess I'd first like to say that chinese water dragons, are just plain awesome. They're fun to own, and you can really deck out their tanks too and make it all foresty and wateryish haha. I wouldn't consider them beginner reptiles though unfortunately. But you seem like you have a lot of experience with reptiles, and I bet you'd do just fine with a water dragon.
I don't know much about the frilled dragons, but they look really neat. I know a guy who had one and just sold it, but he really seemed to like his a lot.
I too would suggest a mali uromastyx. You can get them in really neat morphs if your not into the normal looking color. They're similar to bearded dragons, but I don't find them as common as bearded dragons are, and they are a little unusual with their tail being all spiny and cool color morphs and such. They do require a bit more heat than a bearded dragon, just to give you a general idea.
I find water monitors really cool and very interesting. I've always wanted one, but I'm focusing on snakes at the moment. They're really unusual... well at least to me they are. I know you can take them on walks and swims or whatever and I find that really fascinating, and they can get really big which is awesome and fun.
But out of all these... I'd get the water dragon to start with :)
Here's a link to my water dragon's cage way before I got the dragon and such, maybe it can give ya an idea of how you can set the tank up if you wanted.