a tuxedo urchin can be cool, if you don't mind it eating your corraline somtimes. if you supplement with nori, which you need to do if there's not an abundance of algae in your tank, you might be able to train it off corraline completely. most places don't mention they eat that.
also, they tend to knock over stuff, so make sure your coral frags and rock are secure and are well mounted.
http://www.fishpalace.org/M_globulus.html
http://www.liveaquaria.com/product/prod_Display.cfm?pCatId=595
lobters are also unusual, but usually will do better in a coral/larger fish with live rock tank, as they will hunt small fish and inverts... but they're not the HUGE difficulty that like, scallops/oysters/sand dollars are. you just have to stock your tank accordingly. also with lobsters, usually you need to provide a cave. and they can get big.
http://www.liveaquaria.com/product/scateg.cfm?pCatId=506
i would advise against things like sea apples, sea cucumbers and nudibranches because they can nuke your tank if they die and you don't catch it quickly enough. plus, most have highly specialized diets (ie eat only aiptasia or flatworms) and once that food supply is exhausted, they die.
you can also get some interesting shrimp, the ghost boxer, which is like an albino coral banded, is cool. there are also some interesting crabs.
basically, anything you get that's going to be unique and interesting, is usually that way because it's only semi-reef-safe, and most people don't want to take a chance on it, so not many sell, leading to lower collections and higher rareity. it's up to you... personally, if i had a big tank, and wanted to do something cool, as much as i love shrimp and gobies/smaller fish, i'd ditch them, and the smaller fish, and get a blue lobster. that would be neat. there's a purple one too, but i can't remember how big it gets. just remember, that sucker will end up over a foot, if things go well.
without making changed to your current setup, a tux urchin will probably integrate the easiest, if you don't mind some corraline loss.