Well, I still think that "Gladiator" is a far more flavorful Dark Sun class name than "Fighter" is. I'd gladly just re-label the fighter class as gladiator. Dark Sun fighters have always leaned towards what the Warlord is in 4e.
On the other hand, I'm thinking Bards could probably be rolled into the rogue class pretty easily. Give them a bard like talent tree or paragon path or something. Rogues have become alot more varied since second edition, and what the Dark Sun bard does is well within the capabilities of a modern rogue.
I'd probably also remove druids from the original book, and save them for a supplement. Dark Sun druids never made a whole lot of sense as adventurers. They are religiously dedicated to a specific piece of land. The character just doesn't fit in the standard playstyle that includes alot of travelling. I do like the class alot, but it's non-standard enough that I think it should be optional rather than core.
The biggest change I'd make from third edition Dark Sun would be to give everybody a wild talent again. Not sure how I'd do it(as I obviously have no idea how psionics works in 4e), but it'd probably amount to giving everybody a level 1 per encounter ability from one of the psionic classes(note that I'd even give this to the psionic classes, basically giving them an extra power). I suspect that the reason they decided not to give out free wild talents in 3e is because they were a D&D setting and were mandated to follow the D&D character creation rules. This is another of the reasons why I think making it a standalone game rather than a D&D setting is a good idea, as I think wild talents are an important part of the setting. They shouldn't be 2e style(I've got a first level character with disintegrate!), but they should be there.
One happy change in 4th edition is that races seem to be getting +2 to two attributes, which brings it closer to Dark Sun. Half Giants will still be a problem, and Thri Kreen might be a problem as well, but the powering up of the races in 4e will help in recreating Dark Sun.