Which currently non-core classes would you add?

[Klaus]

Hard to argue with that.

The Archivist is also a good candidate.

I love me some Swashbucklers, but I'm not against just building one with a rogue.

If you downplay the armor (no armor), weapon (wizard weapons plus sickle and scythe) and wildshaping (Tiny, Small and Medium only), the druid could be a very decent stand-in for a witch class. Maybe a "Cloistered Druid"?

 

[Traycor]

Swordmage FTW

 

[Imp]

I love me some Swashbucklers, but I'm not against just building one with a rogue.

You can do the sneaky underhanded scoundrel types that way, and I do that all the time, but the stand-up charismatic ones are the guys that hold off the guards, not the ones running up to 'em and shanking them in the belly while they're focused on Clanky McGreatAxe, which is why I find it insufficient. Melee ranger might be a better fit, but comes with a lot of survivalist garnish, I'd imagine.

As for witches, I actually have made the druid -> witch equivalence in the past with 3E, it seemed a natural fit. I dunno about the 4E implementation though.